libstdc++: Implement P1494 and P3641 Partial program correctness [PR119060]

This implements the library parts of P1494 as amended by P3641.  For GCC the
compiler itself treats stdio operations as equivalent to the observable
checkpoint and thus it does not appear to be necessary to add calls to those
functions (it will not alter the outcome).

This adds the facility for C++26, although there is no reason, in principle,
that it would not work back to C++11 at least.

	PR c++/119060

libstdc++-v3/ChangeLog:

	* include/bits/version.def: Add observable_checkpoint at present
	allowed from C++26.
	* include/bits/version.h: Regenerate.
	* include/std/utility: Add std::observable_checkpoint().
	* src/c++23/std.cc.in: Add obervable_checkpoint () to utility.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/builtins.cc

@@ -8427,6 +8427,10 @@ expand_builtin (tree exp, rtx target, rtx subtarget, machine_mode mode,
       expand_builtin_unreachable ();
       return const0_rtx;
 
+    case BUILT_IN_OBSERVABLE_CHKPT:
+      /* Generate no code.  */
+      return const0_rtx;
+
     CASE_FLT_FN (BUILT_IN_SIGNBIT):
     case BUILT_IN_SIGNBITD32:
     case BUILT_IN_SIGNBITD64:

gcc/builtins.def

@@ -1142,6 +1142,7 @@ DEF_C2Y_BUILTIN        (BUILT_IN_ULABS, "ulabs", BT_FN_ULONG_LONG, ATTR_CONST_NO
 DEF_C2Y_BUILTIN        (BUILT_IN_ULLABS, "ullabs", BT_FN_ULONGLONG_LONGLONG, ATTR_CONST_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_UNREACHABLE_TRAP, "unreachable trap", BT_FN_VOID, ATTR_CONST_NORETURN_NOTHROW_LEAF_COLD_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_UNREACHABLE, "unreachable", BT_FN_VOID, ATTR_CONST_NORETURN_NOTHROW_LEAF_COLD_LIST)
+DEF_GCC_BUILTIN        (BUILT_IN_OBSERVABLE_CHKPT, "observable_checkpoint", BT_FN_VOID, ATTR_NOTHROW_LEAF_LIST)
 DEF_GCC_BUILTIN        (BUILT_IN_UNWIND_INIT, "unwind_init", BT_FN_VOID, ATTR_NULL)
 DEF_GCC_BUILTIN        (BUILT_IN_UPDATE_SETJMP_BUF, "update_setjmp_buf", BT_FN_VOID_PTR, ATTR_NULL)
 DEF_GCC_BUILTIN        (BUILT_IN_VA_COPY, "va_copy", BT_FN_VOID_VALIST_REF_VALIST_ARG, ATTR_NOTHROW_LEAF_LIST)

gcc/cp/cxxapi-data.csv

@@ -952,6 +952,7 @@
 <utility>,in_range,1,cxx20
 <utility>,to_underlying,1,cxx23
 <utility>,unreachable,1,cxx23
+<utility>,observable_checkpoint,1,cxx26
 <utility>,integer_sequence,1,cxx14
 <utility>,index_sequence,1,cxx14
 <utility>,make_integer_sequence,1,cxx14

gcc/cp/std-name-hint.gperf

@@ -577,6 +577,7 @@ piecewise_construct, "<utility>", cxx11
 piecewise_construct_t, "<utility>", cxx11
 to_underlying, "<utility>", cxx23
 unreachable, "<utility>", cxx23
+observable_checkpoint, "<utility>", cxx26
 # <variant>
 bad_variant_access, "<variant>", cxx17
 holds_alternative, "<variant>", cxx17

gcc/cp/std-name-hint.h

@@ -135,7 +135,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
 {
   enum
     {
-      TOTAL_KEYWORDS = 487,
+      TOTAL_KEYWORDS = 488,
       MIN_WORD_LENGTH = 2,
       MAX_WORD_LENGTH = 39,
       MIN_HASH_VALUE = 7,
@@ -178,7 +178,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"print", "<print>", cxx23},
 #line 221 "std-name-hint.gperf"
       {"promise", "<future>", cxx11},
-#line 581 "std-name-hint.gperf"
+#line 582 "std-name-hint.gperf"
       {"bad_variant_access", "<variant>", cxx17},
 #line 328 "std-name-hint.gperf"
       {"to_address", "<memory>", cxx20},
@@ -198,7 +198,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"nounitbuf", "<ios>", cxx98},
 #line 433 "std-name-hint.gperf"
       {"basic_stringbuf", "<sstream>", cxx98},
-#line 592 "std-name-hint.gperf"
+#line 593 "std-name-hint.gperf"
       {"vector", "<vector>", cxx98},
 #line 246 "std-name-hint.gperf"
       {"noshowbase", "<ios>", cxx98},
@@ -228,7 +228,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"tuple_element", "<tuple>", cxx11},
 #line 519 "std-name-hint.gperf"
       {"tuple_element_t", "<tuple>", cxx14},
-#line 584 "std-name-hint.gperf"
+#line 585 "std-name-hint.gperf"
       {"variant", "<variant>", cxx17},
 #line 386 "std-name-hint.gperf"
       {"ends", "<ostream>", cxx98},
@@ -244,11 +244,11 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"noshowpos", "<ios>", cxx98},
 #line 388 "std-name-hint.gperf"
       {"flush_emit", "<ostream>", cxx20},
-#line 585 "std-name-hint.gperf"
+#line 586 "std-name-hint.gperf"
       {"variant_alternative", "<variant>", cxx17},
 #line 537 "std-name-hint.gperf"
       {"void_t", "<type_traits>", cxx17},
-#line 586 "std-name-hint.gperf"
+#line 587 "std-name-hint.gperf"
       {"variant_alternative_t", "<variant>", cxx17},
 #line 134 "std-name-hint.gperf"
       {"relation", "<concepts>", cxx20},
@@ -338,7 +338,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"basic_streambuf", "<streambuf>", cxx98},
 #line 167 "std-name-hint.gperf"
       {"basic_format_args", "<format>", cxx20},
-#line 588 "std-name-hint.gperf"
+#line 589 "std-name-hint.gperf"
       {"variant_size", "<variant>", cxx17},
 #line 407 "std-name-hint.gperf"
       {"multiset", "<set>", cxx98},
@@ -360,13 +360,13 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"remove_cvref_t", "<type_traits>", cxx20},
 #line 289 "std-name-hint.gperf"
       {"ostream_iterator", "<iterator>", cxx98},
-#line 587 "std-name-hint.gperf"
+#line 588 "std-name-hint.gperf"
       {"variant_npos", "<variant>", cxx17},
 #line 244 "std-name-hint.gperf"
       {"left", "<ios>", cxx98},
 #line 200 "std-name-hint.gperf"
       {"fstream", "<fstream>", cxx98},
-#line 590 "std-name-hint.gperf"
+#line 591 "std-name-hint.gperf"
       {"visit", "<variant>", cxx17},
 #line 208 "std-name-hint.gperf"
       {"invoke", "<functional>", cxx17},
@@ -458,7 +458,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"showpoint", "<ios>", cxx98},
 #line 446 "std-name-hint.gperf"
       {"stacktrace", "<stacktrace>", cxx23},
-#line 589 "std-name-hint.gperf"
+#line 590 "std-name-hint.gperf"
       {"variant_size_v", "<variant>", cxx17},
 #line 212 "std-name-hint.gperf"
       {"reference_wrapper", "<functional>", cxx11},
@@ -474,7 +474,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"allocator_traits", "<memory>", cxx11},
 #line 183 "std-name-hint.gperf"
       {"make_wformat_args", "<format>", cxx20},
-#line 583 "std-name-hint.gperf"
+#line 584 "std-name-hint.gperf"
       {"monostate", "<variant>", cxx17},
 #line 387 "std-name-hint.gperf"
       {"flush", "<ostream>", cxx98},
@@ -582,7 +582,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"stop_source", "<stop_token>", cxx20},
 #line 548 "std-name-hint.gperf"
       {"unordered_set", "<unordered_set>", cxx11},
-#line 582 "std-name-hint.gperf"
+#line 583 "std-name-hint.gperf"
       {"holds_alternative", "<variant>", cxx17},
 #line 514 "std-name-hint.gperf"
       {"make_tuple", "<tuple>", cxx11},
@@ -960,6 +960,8 @@ std_name_hint_lookup::find (const char *str, size_t len)
       {"domain_error", "<stdexcept>", cxx98},
 #line 84 "std-name-hint.gperf"
       {"chrono::steady_clock", "<chrono>", cxx11},
+#line 580 "std-name-hint.gperf"
+      {"observable_checkpoint", "<utility>", cxx26},
 #line 188 "std-name-hint.gperf"
       {"vformat_to", "<format>", cxx20},
 #line 54 "std-name-hint.gperf"
@@ -1142,7 +1144,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,    24,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    25,    26,    27,    -1,
          -1,    -1,    28,    29,    -1,    -1,    30,
-       -629,  -456,    -2,    33,    -1,    34,    -1,
+       -630,  -457,    -2,    33,    -1,    34,    -1,
          35,    -1,    36,    -1,    -1,    -1,    -1,
          37,    -1,    38,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    39,    -1,    -1,    -1,
@@ -1184,8 +1186,8 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,   152,   153,    -1,    -1,    -1,    -1,
          -1,    -1,   154,    -1,   155,   156,    -1,
          -1,    -1,   157,    -1,    -1,   158,    -1,
-        159,   160,    -1,    -1,    -1,  -933,    -1,
+        159,   160,    -1,    -1,    -1,  -934,    -1,
-        163,   164,    -1,   165,  -326,    -2,    -1,
+        163,   164,    -1,   165,  -327,    -2,    -1,
          -1,   166,    -1,    -1,    -1,    -1,   167,
         168,    -1,   169,    -1,    -1,    -1,   170,
         171,   172,   173,    -1,    -1,    -1,   174,
@@ -1195,11 +1197,11 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,   185,    -1,   186,   187,    -1,   188,
         189,   190,   191,   192,    -1,   193,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   194,    -1, -1005,  -292,
+         -1,    -1,    -1,   194,    -1, -1006,  -293,
          -2,    -1,    -1,    -1,   197,    -1,    -1,
         198,   199,   200,   201,    -1,    -1,    -1,
         202,   203,    -1,    -1,    -1,    -1,   204,
-         -1,    -1, -1030,  -282,    -2,    -1,   207,
+         -1,    -1, -1031,  -283,    -2,    -1,   207,
          -1,    -1,    -1,   208,   209,    -1,    -1,
          -1,   210,    -1,    -1,    -1,   211,    -1,
         212,    -1,    -1,   213,    -1,    -1,   214,
@@ -1227,7 +1229,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
         275,   276,    -1,   277,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,   278,    -1,
          -1,    -1,    -1,    -1,   279,   280,   281,
-      -1225,   284,  -205,    -2,    -1,    -1,    -1,
+      -1226,   284,  -206,    -2,    -1,    -1,    -1,
          -1,   285,    -1,    -1,    -1,    -1,   286,
         287,   288,    -1,    -1,   289,   290,   291,
          -1,    -1,    -1,    -1,   292,    -1,    -1,
@@ -1247,7 +1249,7 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,   331,    -1,   332,    -1,    -1,
          -1,    -1,   333,   334,   335,    -1,    -1,
          -1,   336,    -1,    -1,   337,   338,    -1,
-         -1,    -1,    -1, -1367,  -148,    -2,    -1,
+         -1,    -1,    -1, -1368,  -149,    -2,    -1,
         341,    -1,    -1,    -1,    -1,    -1,    -1,
         342,    -1,    -1,   343,    -1,    -1,    -1,
          -1,   344,   345,    -1,   346,    -1,    -1,
@@ -1282,70 +1284,70 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,    -1,    -1,    -1,    -1,   395,
         396,   397,    -1,    -1,   398,   399,    -1,
         400,   401,   402,   403,   404,   405,   406,
-        407,    -1,   408,    -1,    -1,   409,   410,
+        407,   408,   409,    -1,    -1,   410,   411,
-         -1,    -1,    -1,   411,    -1,    -1,    -1,
+         -1,    -1,    -1,   412,    -1,    -1,    -1,
-         -1,   412,    -1,   413,    -1,    -1,   414,
+         -1,   413,    -1,   414,    -1,    -1,   415,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,   415,    -1,    -1,    -1,   416,    -1,
+         -1,   416,    -1,    -1,    -1,   417,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-        417,    -1,    -1,   418,    -1,    -1,   419,
+        418,    -1,    -1,   419,    -1,    -1,   420,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   420,   421,    -1,    -1,
+         -1,    -1,    -1,   421,   422,    -1,    -1,
-         -1,    -1,    -1,    -1,   422,    -1,    -1,
          -1,    -1,    -1,    -1,   423,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,   424,    -1,    -1,
-         -1,    -1,    -1,    -1, -1704,   -62,    -2,
-         -1,    -1,    -1,    -1,   427,    -1,    -1,
-         -1,    -1,    -1,   428,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,   429,    -1,    -1,
+         -1,    -1,    -1,    -1,   425,    -1,    -1,
+         -1,    -1,    -1,    -1, -1705,   -62,    -2,
+         -1,    -1,    -1,    -1,   428,    -1,    -1,
+         -1,    -1,    -1,   429,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-        430,    -1,    -1,    -1,    -1,    -1,   431,
+         -1,    -1,    -1,    -1,   430,    -1,    -1,
-         -1,    -1,    -1,    -1,   432,    -1,    -1,
+         -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,   433,    -1,
+        431,    -1,    -1,    -1,    -1,    -1,   432,
-         -1,    -1,    -1,   434,    -1,    -1,   435,
+         -1,    -1,    -1,    -1,   433,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,    -1,   436,
+         -1,    -1,    -1,    -1,    -1,   434,    -1,
-        437,    -1,    -1,   438,    -1,    -1,    -1,
+         -1,    -1,    -1,   435,    -1,    -1,   436,
-         -1,    -1,   439,   440,    -1,    -1,    -1,
+         -1,    -1,    -1,    -1,    -1,    -1,   437,
-        441,    -1,   442,    -1,   443,    -1,    -1,
+        438,    -1,    -1,   439,    -1,    -1,    -1,
-        444,    -1,    -1,    -1,   445,    -1,    -1,
+         -1,    -1,   440,   441,    -1,    -1,    -1,
+        442,    -1,   443,    -1,   444,    -1,    -1,
+        445,    -1,    -1,    -1,   446,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,    -1,   446,
+         -1,    -1,    -1,    -1,    -1,    -1,   447,
-         -1,   447,    -1,    -1,    -1,    -1,    -1,
+         -1,   448,    -1,    -1,    -1,    -1,    -1,
-        448,    -1,    -1,    -1,    -1,    -1,    -1,
+        449,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   449,    -1,   450,    -1,
+         -1,    -1,    -1,   450,    -1,   451,    -1,
-         -1,    -1,    -1,    -1,   451,    -1,    -1,
+         -1,    -1,    -1,    -1,   452,    -1,    -1,
-         -1,    -1,    -1,   452,    -1,    -1,    -1,
+         -1,    -1,    -1,   453,    -1,    -1,    -1,
-         -1,    -1,   453,    -1,    -1,   454,    -1,
+         -1,    -1,   454,    -1,    -1,   455,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,   455,    -1,    -1,    -1,   456,    -1,
+         -1,   456,    -1,    -1,    -1,   457,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   457,    -1,   458,   459,
+         -1,    -1,    -1,   458,    -1,   459,   460,
-         -1,    -1,    -1,    -1,    -1,    -1,   460,
+         -1,    -1,    -1,    -1,    -1,    -1,   461,
-         -1,    -1,    -1,    -1,   461,    -1,    -1,
+         -1,    -1,    -1,    -1,   462,    -1,    -1,
-         -1,    -1,   462,    -1,    -1,    -1,    -1,
+         -1,    -1,   463,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,   463,    -1,    -1,
+         -1,    -1,    -1,    -1,   464,    -1,    -1,
-         -1,    -1,    -1,   464,    -1,    -1,    -1,
          -1,    -1,    -1,   465,    -1,    -1,    -1,
+         -1,    -1,    -1,   466,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,   466,    -1,    -1,
+         -1,    -1,    -1,    -1,   467,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,   467,    -1,    -1,    -1,   468,
+         -1,    -1,   468,    -1,    -1,    -1,   469,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,    -1,   469,
+         -1,    -1,    -1,    -1,    -1,    -1,   470,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
@@ -1354,22 +1356,22 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,   470,    -1,    -1,    -1,    -1,    -1,
+         -1,   471,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   471,    -1,   472,    -1,
+         -1,    -1,    -1,   472,    -1,   473,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,   473,    -1,    -1,    -1,    -1,
+         -1,    -1,   474,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,   474,    -1,
+         -1,    -1,    -1,    -1,    -1,   475,    -1,
-         -1,   475,    -1,    -1,    -1,    -1,    -1,
+         -1,   476,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-        476,    -1,    -1,    -1,    -1,    -1,    -1,
+        477,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
@@ -1380,14 +1382,14 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,   477,    -1,    -1,
+         -1,    -1,    -1,    -1,   478,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   478,   479,   480,    -1,
+         -1,    -1,    -1,   479,   480,   481,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,   481,    -1,
+         -1,    -1,    -1,    -1,    -1,   482,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
@@ -1402,12 +1404,12 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,   482,    -1,    -1,    -1,    -1,    -1,
+         -1,   483,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,   483,    -1,    -1,    -1,
+         -1,    -1,    -1,   484,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
@@ -1415,12 +1417,12 @@ std_name_hint_lookup::find (const char *str, size_t len)
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-         -1,    -1,    -1,    -1,    -1,   484,    -1,
          -1,    -1,    -1,    -1,    -1,   485,    -1,
+         -1,    -1,    -1,    -1,    -1,   486,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
          -1,    -1,    -1,    -1,    -1,    -1,    -1,
-        486
+        487
     };
 
   if (len <= MAX_WORD_LENGTH && len >= MIN_WORD_LENGTH)

gcc/testsuite/g++.dg/cpp26/observable-checkpoint.C

@@ -0,0 +1,25 @@
+// P1494R5+P3641R0 - Partial program correctness.
+// { dg-do compile { target c++11 } }
+// { dg-additional-options "-fdump-tree-optimized -Wno-return-type -O" }
+// { dg-final { scan-tree-dump  {\+\s42} "optimized" } }
+// { dg-final { scan-tree-dump  {__builtin_observable_checkpoint} "optimized" } }
+
+extern int x;
+
+int
+here_b_ub ()
+{
+  // missing return triggers UB (we must ignore the warning for this test). 
+}
+
+int
+main ()
+{
+  x = 0;
+  __builtin_printf (" start \n");
+  x += 42;
+  // Without this checkpoint the addition above is elided (along with the rest
+  // of main).
+  __builtin_observable_checkpoint ();
+  here_b_ub ();
+}

gcc/tree.cc

@@ -10012,7 +10012,7 @@ set_call_expr_flags (tree decl, int flags)
     /* Looping const or pure is implied by noreturn.
      There is currently no way to declare looping const or looping pure alone.  */
   gcc_assert (!(flags & ECF_LOOPING_CONST_OR_PURE)
-	      || ((flags & ECF_NORETURN) && (flags & (ECF_CONST | ECF_PURE))));
+	      || (flags & (ECF_CONST | ECF_PURE)));
 }
 
 
@@ -10057,6 +10057,7 @@ build_common_builtin_nodes (void)
   if (!builtin_decl_explicit_p (BUILT_IN_UNREACHABLE)
       || !builtin_decl_explicit_p (BUILT_IN_TRAP)
       || !builtin_decl_explicit_p (BUILT_IN_UNREACHABLE_TRAP)
+      || !builtin_decl_explicit_p (BUILT_IN_OBSERVABLE_CHKPT)
       || !builtin_decl_explicit_p (BUILT_IN_ABORT))
     {
       ftype = build_function_type (void_type_node, void_list_node);
@@ -10080,6 +10081,12 @@ build_common_builtin_nodes (void)
 	local_define_builtin ("__builtin_trap", ftype, BUILT_IN_TRAP,
 			      "__builtin_trap",
 			      ECF_NORETURN | ECF_NOTHROW | ECF_LEAF | ECF_COLD);
+      if (!builtin_decl_explicit_p (BUILT_IN_OBSERVABLE_CHKPT))
+	local_define_builtin ("__builtin_observable_checkpoint", ftype,
+			      BUILT_IN_OBSERVABLE_CHKPT,
+			      "__builtin_observable_checkpoint",
+			      ECF_NOTHROW | ECF_LEAF | ECF_CONST
+			      | ECF_LOOPING_CONST_OR_PURE);
     }
 
   if (!builtin_decl_explicit_p (BUILT_IN_MEMCPY)

libstdc++-v3/include/bits/version.def

@@ -1950,6 +1950,15 @@ ftms = {
   };
 };
 
+ftms = {
+  name = observable_checkpoint;
+  values = {
+    v = 202506;
+    cxxmin = 26;
+    extra_cond = "__has_builtin(__builtin_observable_checkpoint)";
+  };
+};
+
 ftms = {
   name = algorithm_default_value_type;
   values = {

libstdc++-v3/include/bits/version.h

@@ -2181,6 +2181,16 @@
 #endif /* !defined(__cpp_lib_unreachable) */
 #undef __glibcxx_want_unreachable
 
+#if !defined(__cpp_lib_observable_checkpoint)
+# if (__cplusplus >  202302L) && (__has_builtin(__builtin_observable_checkpoint))
+#  define __glibcxx_observable_checkpoint 202506L
+#  if defined(__glibcxx_want_all) || defined(__glibcxx_want_observable_checkpoint)
+#   define __cpp_lib_observable_checkpoint 202506L
+#  endif
+# endif
+#endif /* !defined(__cpp_lib_observable_checkpoint) && defined(__glibcxx_want_observable_checkpoint) */
+#undef __glibcxx_want_observable_checkpoint
+
 #if !defined(__cpp_lib_algorithm_default_value_type)
 # if (__cplusplus >  202302L)
 #  define __glibcxx_algorithm_default_value_type 202403L

libstdc++-v3/include/std/utility

@@ -98,6 +98,7 @@
 #define __glibcxx_want_tuple_element_t
 #define __glibcxx_want_tuples_by_type
 #define __glibcxx_want_unreachable
+#define __glibcxx_want_observable_checkpoint
 #define __glibcxx_want_tuple_like
 #define __glibcxx_want_constrained_equality
 #include <bits/version.h>
@@ -234,6 +235,24 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
   }
 #endif
 
+
+#ifdef __cpp_lib_observable_checkpoint // C++ >= 26
+  /// Informs the compiler that prior actions are considered observable.
+  /**
+   * This may be used to limit the extent to which optimisations based on the
+   * assumed unreachability of undefined behaviour can propagate to earlier
+   * code.
+   *
+   * @since C++26
+   */
+  [[__gnu__::__always_inline__]]
+  inline void
+  observable_checkpoint() noexcept
+  {
+    return __builtin_observable_checkpoint();
+  }
+#endif
+
 _GLIBCXX_END_NAMESPACE_VERSION
 } // namespace
 

libstdc++-v3/src/c++23/std.cc.in

@@ -3337,6 +3337,9 @@ export namespace std
 #if __cpp_lib_unreachable
   using std::unreachable;
 #endif
+#if __cpp_lib_observable_checkpoint
+  using std::observable_checkpoint;
+#endif
 }
 
 // <valarray>