diff --git a/gcc/c-family/c-lex.cc b/gcc/c-family/c-lex.cc
index f7168ce6fc04..8e1829389385 100644
--- a/gcc/c-family/c-lex.cc
+++ b/gcc/c-family/c-lex.cc
@@ -1347,7 +1347,12 @@ interpret_float (const cpp_token *token, unsigned int flags,
   if (flags & CPP_N_USERDEF)
     copylen -= strlen (suffix);
   else if (flags & CPP_N_DFLOAT)
-    copylen -= 2;
+    {
+      if (ISDIGIT (token->val.str.text[copylen - 1]))
+	copylen -= (flags & CPP_N_LARGE) ? 4 : 3;
+      else
+	copylen -= 2;
+    }
   else
     {
       if ((flags & CPP_N_WIDTH) != CPP_N_MEDIUM)
diff --git a/gcc/testsuite/gcc.dg/dfp/c11-constants-3.c b/gcc/testsuite/gcc.dg/dfp/c11-constants-3.c
new file mode 100644
index 000000000000..63b9a71b8aea
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/dfp/c11-constants-3.c
@@ -0,0 +1,13 @@
+/* Test that DFP constants are diagnosed in C11 mode: -pedantic.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic" } */
+
+int a = (int) 1.1D32; /* { dg-warning "C23 feature" } */
+int b = (int) 2.d32; /* { dg-warning "C23 feature" } */
+int c = (int) .33D64; /* { dg-warning "C23 feature" } */
+int d = (int) 2e1d64; /* { dg-warning "C23 feature" } */
+int e = (int) .3e2D128; /* { dg-warning "C23 feature" } */
+int f = (int) 4.5e3d128; /* { dg-warning "C23 feature" } */
+int g = (int) 5.e0D32; /* { dg-warning "C23 feature" } */
+int h = (int) 1e+2d32; /* { dg-warning "C23 feature" } */
+int i = (int) 1000e-3D128; /* { dg-warning "C23 feature" } */
diff --git a/gcc/testsuite/gcc.dg/dfp/c11-constants-4.c b/gcc/testsuite/gcc.dg/dfp/c11-constants-4.c
new file mode 100644
index 000000000000..09e2ae1bdd31
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/dfp/c11-constants-4.c
@@ -0,0 +1,13 @@
+/* Test that DFP constants are diagnosed in C11 mode: -pedantic-errors.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c11 -pedantic-errors" } */
+
+int a = (int) 1.1D32; /* { dg-error "C23 feature" } */
+int b = (int) 2.d32; /* { dg-error "C23 feature" } */
+int c = (int) .33D64; /* { dg-error "C23 feature" } */
+int d = (int) 2e1d64; /* { dg-error "C23 feature" } */
+int e = (int) .3e2D128; /* { dg-error "C23 feature" } */
+int f = (int) 4.5e3d128; /* { dg-error "C23 feature" } */
+int g = (int) 5.e0D32; /* { dg-error "C23 feature" } */
+int h = (int) 1e+2d32; /* { dg-error "C23 feature" } */
+int i = (int) 1000e-3D128; /* { dg-error "C23 feature" } */
diff --git a/gcc/testsuite/gcc.dg/dfp/c23-constants-3.c b/gcc/testsuite/gcc.dg/dfp/c23-constants-3.c
new file mode 100644
index 000000000000..7b09715a4bb4
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/dfp/c23-constants-3.c
@@ -0,0 +1,39 @@
+/* Test that DFP constants are accepted in C23 mode.  */
+/* { dg-do run } */
+/* { dg-options "-std=c23 -pedantic-errors" } */
+
+int a = (int) 1.1D32;
+int b = (int) 2.d32;
+int c = (int) .33D64;
+int d = (int) 2e1d64;
+int e = (int) .3e2D128;
+int f = (int) 4.5e3d128;
+int g = (int) 5.e0D32;
+int h = (int) 1e+2d32;
+int i = (int) 1000e-3D128;
+
+#define expr_has_type(e, t) _Generic (e, default : 0, t : 1)
+static_assert (expr_has_type (1.1D32, _Decimal32));
+static_assert (expr_has_type (2.d32, _Decimal32));
+static_assert (expr_has_type (.33D64, _Decimal64));
+static_assert (expr_has_type (2e1d64, _Decimal64));
+static_assert (expr_has_type (.3e2D128, _Decimal128));
+static_assert (expr_has_type (4.5e3d128, _Decimal128));
+static_assert (expr_has_type (5.e0D32, _Decimal32));
+static_assert (expr_has_type (1e+2d32, _Decimal32));
+static_assert (expr_has_type (1000e-3D128, _Decimal128));
+
+int
+main ()
+{
+  if (1.1D32 != 1.1df
+      || 2.d32 != 2.df
+      || .33D64 != .33dd
+      || 2e1d64 != 2e1dd
+      || .3e2D128 != .3e2dl
+      || 4.5e3d128 != 4.5e3dl
+      || 5.e0D32 != 5.e0df
+      || 1e+2d32 != 1e+2df
+      || 1000e-3D128 != 1000e-3dl)
+    __builtin_abort ();
+}
diff --git a/gcc/testsuite/gcc.dg/dfp/c23-constants-4.c b/gcc/testsuite/gcc.dg/dfp/c23-constants-4.c
new file mode 100644
index 000000000000..e9ae6a375d50
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/dfp/c23-constants-4.c
@@ -0,0 +1,13 @@
+/* Test that DFP constants are accepted in C23 mode: compat warnings.  */
+/* { dg-do compile } */
+/* { dg-options "-std=c23 -Wc11-c23-compat" } */
+
+int a = (int) 1.1D32; /* { dg-warning "C23 feature" } */
+int b = (int) 2.d32; /* { dg-warning "C23 feature" } */
+int c = (int) .33D64; /* { dg-warning "C23 feature" } */
+int d = (int) 2e1d64; /* { dg-warning "C23 feature" } */
+int e = (int) .3e2D128; /* { dg-warning "C23 feature" } */
+int f = (int) 4.5e3d128; /* { dg-warning "C23 feature" } */
+int g = (int) 5.e0D32; /* { dg-warning "C23 feature" } */
+int h = (int) 1e+2d32; /* { dg-warning "C23 feature" } */
+int i = (int) 1000e-3D128; /* { dg-warning "C23 feature" } */
diff --git a/libcpp/expr.cc b/libcpp/expr.cc
index bbf21b8fb1c3..685233797c84 100644
--- a/libcpp/expr.cc
+++ b/libcpp/expr.cc
@@ -99,9 +99,9 @@ interpret_float_suffix (cpp_reader *pfile, const uchar *s, size_t len)
   /* The following decimal float suffixes, from TR 24732:2009, TS
      18661-2:2015 and C23, are supported:
 
-     df, DF - _Decimal32.
-     dd, DD - _Decimal64.
-     dl, DL - _Decimal128.
+     df, DF, d32, D32 - _Decimal32.
+     dd, DD, d64, D64 - _Decimal64.
+     dl, DL, d128, D128 - _Decimal128.
 
      The dN and DN suffixes for _DecimalN, and dNx and DNx for
      _DecimalNx, defined in TS 18661-3:2015, are not supported.
@@ -253,7 +253,18 @@ interpret_float_suffix (cpp_reader *pfile, const uchar *s, size_t len)
 	      break;
 	    }
 	  return 0;
-	case 'd': case 'D': d++; break;
+	case 'd': case 'D':
+	  if (!CPP_OPTION (pfile, cplusplus) && orig_s == s && len > 1)
+	    {
+	      if (s[1] == '3' && s[2] == '2' && len == 2)
+		return CPP_N_DFLOAT | CPP_N_SMALL;
+	      if (s[1] == '6' && s[2] == '4' && len == 2)
+		return CPP_N_DFLOAT | CPP_N_MEDIUM;
+	      if (s[1] == '1' && s[2] == '2' && len == 3 && s[3] == '8')
+		return CPP_N_DFLOAT | CPP_N_LARGE;
+	    }
+	  d++;
+	  break;
 	case 'l': case 'L': l++; break;
 	case 'w': case 'W': w++; break;
 	case 'q': case 'Q': q++; break;