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| author | David Wohlferd <dw@LimeGreenSocks.com> | 2016-03-18 19:12:10 +0000 |
|---|---|---|
| committer | Bernd Schmidt <bernds@gcc.gnu.org> | 2016-03-18 19:12:10 +0000 |
| commit | 0249ef0bfafd5c8ffc2fcf368199dd62b91ea55e (patch) | |
| tree | e19b730f7870825336c7856d8152ce2f26a4793c /gcc/doc | |
| parent | 5c6a601cd2eac3730d4e00a0c1fe0b3cdb5fc1b4 (diff) | |
| download | gcc-0249ef0bfafd5c8ffc2fcf368199dd62b91ea55e.zip gcc-0249ef0bfafd5c8ffc2fcf368199dd62b91ea55e.tar.gz gcc-0249ef0bfafd5c8ffc2fcf368199dd62b91ea55e.tar.bz2 | |
David Wohlferd's patch to document more issues with basic asms
* doc/extend.texi: Document more potential problems with basic asms.
From-SVN: r234343
Diffstat (limited to 'gcc/doc')
| -rw-r--r-- | gcc/doc/extend.texi | 29 |
1 files changed, 22 insertions, 7 deletions
diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 8fddb34..623a5d0 100644 --- a/gcc/doc/extend.texi +++ b/gcc/doc/extend.texi @@ -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 |