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Re: AW: basic asm and memory clobbers - Proposed solution
- From: David Wohlferd <dw at LimeGreenSocks dot com>
- To: Bernd Edlinger <bernd dot edlinger at hotmail dot de>, "gcc at gcc dot gnu dot org" <gcc at gcc dot gnu dot org>
- Cc: Joseph Myers <joseph at codesourcery dot com>, "Paul_Koning at Dell dot com" <Paul_Koning at Dell dot com>, "jakub at redhat dot com" <jakub at redhat dot com>, "rth at gcc dot gnu dot org" <rth at gcc dot gnu dot org>, "rth at redhat dot com" <rth at redhat dot com>, "pinskia at gcc dot gnu dot org" <pinskia at gcc dot gnu dot org>, Jeff Law <law at redhat dot com>, Segher Boessenkool <segher at kernel dot crashing dot org>, "aph at redhat dot com" <aph at redhat dot com>, Ian Lance Taylor <iant at google dot com>, Sandra Loosemore <sandra at codesourcery dot com>, Hans-Peter Nilsson <hp at bitrange dot com>
- Date: Fri, 27 Nov 2015 23:24:25 -0800
- Subject: Re: AW: basic asm and memory clobbers - Proposed solution
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On 11/27/2015 11:02 PM, Bernd Edlinger wrote:
Hi,
I just found this in the docs:
The compiler copies the assembler instructions in a basic @code{asm}
verbatim to the assembly language output file, without
processing dialects or any of the @samp{%} operators that are available with
extended @code{asm}. This results in minor differences between basic
@code{asm} strings and extended @code{asm} templates. For example, to refer to
registers you might use @samp{%eax} in basic @code{asm} and
@samp{%%eax} in extended @code{asm}.
So it might be good to warn about % in asm statements too, because changing
anything on the asm syntax can be is quite dangerous.
There are a few differences to be aware of between basic and extended,
and yes that is one of them.
I'm putting together a "How to convert basic asm to extended asm"
guide. Not quite sure where to put it yet. Doesn't really belong in
the User Guide, but I worry no one would ever see it in the wiki.
And I wonder what the exact equivalence of asm("") is
in extended asm ("":::) or asm volatile ("":::) ?
From the docs: "asm statements that have no output operands, including
asm goto statements, are implicitly volatile." In other words, those 2
are the same.
Well, I start to think that Jeff is right, and we should treat a asm ("") as if it
were asm volatile ("" ::: )
I believe Segher is already looking at this:
https://gcc.gnu.org/ml/gcc/2015-11/msg00196.html
but if the asm ("nonempty with optional %") we should
treat it as asm volatile ("nonempty with optional %%" ::: "memory").
Our docs should say that explicitly, and the implementation should follow that
direction.
Bernd.