From: David Wohlferd Date: Fri, 18 Mar 2016 19:12:10 +0000 (+0000) Subject: David Wohlferd's patch to document more issues with basic asms X-Git-Url: https://git.libre-soc.org/?a=commitdiff_plain;h=0249ef0bfafd5c8ffc2fcf368199dd62b91ea55e;p=gcc.git David Wohlferd's patch to document more issues with basic asms * doc/extend.texi: Document more potential problems with basic asms. From-SVN: r234343 --- diff --git a/gcc/ChangeLog b/gcc/ChangeLog index 328fde839dd..9aee1f0f834 100644 --- a/gcc/ChangeLog +++ b/gcc/ChangeLog @@ -1,3 +1,8 @@ +2016-03-16 David Wohlferd + Bernd Schmidt + + * doc/extend.texi: Document more potential problems with basic asms. + 2016-03-18 Bernd Schmidt PR rtl-optimization/70278 @@ -42,7 +47,7 @@ define_constraint for "Q" and "T" constraints. 2016-03-17 Evandro Menezes - + Tweak the pipeline model for Exynos M1 * config/aarch64/aarch64.c (exynosm1_tunings): Enable weak prefetching @@ -227,19 +232,19 @@ PR rtl-optimization/69032 * sel-sched-ir.c (get_seqno_by_preds): Include both insn and head when - looping backwards over basic block insns. + looping backwards over basic block insns. 2016-03-15 Andrey Belevantsev PR target/66660 * sel-sched-ir.c (merge_expr): Avoid changing the speculative pattern - to non-speculative when propagating trap bits. + to non-speculative when propagating trap bits. 2016-03-15 Andrey Belevantsev PR rtl-optimization/63384 * sel-sched.c (invoke_aftermath_hooks): Do not decrease issue_more on - DEBUG_INSN_P insns. + DEBUG_INSN_P insns. 2016-03-15 Andrey Belevantsev diff --git a/gcc/doc/extend.texi b/gcc/doc/extend.texi index 8fddb34a7e3..623a5d0c85b 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