Re: [PATCH] Make basic asm implicitly clobber memory

[David Wohlferd <dw at LimeGreenSocks dot com>]

On 5/5/2016 10:29 AM, Bernd Edlinger wrote:
> Hi!
>
> this patch is inspired by recent discussion about basic asm:
>
> Currently a basic asm is an instruction scheduling barrier,
> but not a memory barrier, and most surprising, basic asm
> does _not_ implicitly clobber CC on targets where
> extended asm always implicitly clobbers CC, even if
> nothing is in the clobber section.
>
> This patch makes basic asm implicitly clobber CC on certain
> targets, and makes the basic asm implicitly clobber memory,
> but no general registers, which is what could be expected.
>
> This is however only done for basic asm with non-empty
> assembler string, which is in sync with David's proposed
> basic asm warnings patch.
>
> Due to the change in the tree representation, where
> ASM_INPUT can now be the first element of a
> PARALLEL block with the implicit clobber elements,
> there are some changes necessary.
>
> Most of the changes in the middle end, were necessary
> because extract_asm_operands can not be used to find out
> if a PARALLEL block is an asm statement, but in most cases
> asm_noperands can be used instead.
>
> There are also changes necessary in two targets: pa, and ia64.
> I have successfully built cross-compilers for these targets.
>
> Boot-strapped and reg-tested on x86_64-pc-linux-gnu
> OK for trunk?

A few questions:

1) I'm not clear precisely what problem this patch fixes.  It's true 
that some people have incorrectly assumed that basic asm clobbers memory 
and this change would fix their code.  But some people also incorrectly 
assume it clobbers registers.  I assume that's why Jeff Law proposed 
making basic asm "an opaque blob that read/write/clobber any register or 
memory location."  Do we have enough problem reports from users to know 
which is the real solution here?

2) The -Wbasic-asm warning patch wasn't approved for v6.  If we are 
going to change this behavior now, is it time?

3) I assume there are good reasons why extended asm can't be used at top 
level.  Will adding these clobbers cause those problems in basic asm too?

4) There are more basic asm docs that need to change: "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."

dw