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David Wohlferd's patch to document more issues with basic asms 

* doc/extend.texi: Document more potential problems with basic asms.

From-SVN: r234343
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David Wohlferd 2016-03-18 19:12:10 +00:00 committed by Bernd Schmidt
parent 5c6a601cd2
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2 changed files with 31 additions and 11 deletions
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@ -1,3 +1,8 @@
2016-03-16 David Wohlferd <dw@LimeGreenSocks.com>
Bernd Schmidt <bschmidt@redhat.com>
* doc/extend.texi: Document more potential problems with basic asms.
2016-03-18 Bernd Schmidt <bschmidt@redhat.com>
PR rtl-optimization/70278

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gcc/doc/extend.texi
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@ -7452,10 +7452,10 @@ note that some assembler dialects use semicolons to start a comment.
@end table
@subsubheading Remarks
Using extended @code{asm} typically produces smaller, safer, and more
efficient code, and in most cases it is a better solution than basic
@code{asm}. However, there are two situations where only basic @code{asm}
can be used:
Using extended @code{asm} (@pxref{Extended Asm}) typically produces
smaller, safer, and more efficient code, and in most cases it is a
better solution than basic @code{asm}. However, there are two
situations where only basic @code{asm} can be used:
@itemize @bullet
@item
@ -7492,9 +7492,24 @@ assembly code when optimizing. This can lead to unexpected duplicate
symbol errors during compilation if your assembly code defines symbols or
labels.
Since GCC does not parse the @var{AssemblerInstructions}, it has no
visibility of any symbols it references. This may result in GCC discarding
those symbols as unreferenced.
@strong{Warning:} The C standards do not specify semantics for @code{asm},
making it a potential source of incompatibilities between compilers. These
incompatibilities may not produce compiler warnings/errors.
GCC does not parse basic @code{asm}'s @var{AssemblerInstructions}, which
means there is no way to communicate to the compiler what is happening
inside them. GCC has no visibility of symbols in the @code{asm} and may
discard them as unreferenced. It also does not know about side effects of
the assembler code, such as modifications to memory or registers. Unlike
some compilers, GCC assumes that no changes to either memory or registers
occur. This assumption may change in a future release.
To avoid complications from future changes to the semantics and the
compatibility issues between compilers, consider replacing basic @code{asm}
with extended @code{asm}. See
@uref{https://gcc.gnu.org/wiki/ConvertBasicAsmToExtended, How to convert
from basic asm to extended asm} for information about how to perform this
conversion.
The compiler copies the assembler instructions in a basic @code{asm}
verbatim to the assembly language output file, without