[PATCH] x86: allow to suppress default clobbers added to asm()s

[Jeff Law]

On 10/31/14 02:07, Jan Beulich wrote:
> While it always seemed wrong to me that there's no way to avoid the
> default "flags" and "fpsr" clobbers, the regression the fix for
> PR/60663 introduced (see PR/63637) makes it even more desirable to have
> such a mechanism: This way, at least asm()s with a single output and no
> explicit clobbers can again be made subject to CSE.
>
> gcc:
> 2014-10-31  Jan Beulich  <jbeulich@suse.com>
>
> 	* config/i386/i386.c (ix86_target_string): Add
> 	-mno-default-asm-clobbers.
> 	(ix86_valid_target_attribute_inner_p): Handle
> 	-m{,no-}default-asm-clobbers.
> 	(ix86_md_asm_clobbers): Handle "inverse" clobbers.
> 	* config/i386/i386.h (NOCC_REGNUM, NOFPSR_REGNUM): Define.
> 	(ADDITIONAL_REGISTER_NAMES): Add "cc", "!cc", "!flags", and
> 	"!fpsr".
> 	* config/i386/i386.opt: Add mdefault-asm-clobbers and
> 	mno-default-asm-clobbers.
> 	* varasm.c (decode_reg_name_and_count): Permit negative
> 	register numbers in ADDITIONAL_REGISTER_NAMES.
>
> gcc/testsuite:
> 2014-10-31  Jan Beulich  <jbeulich@suse.com>
>
> 	* gcc.target/i386/20060218-1.c: Adjust expected error.
> 	* gcc.target/i386/invclbr[123].c: New.
I really don't like having an option that's globally applied for this 
feature.  THough I am OK with having a mechanism to avoid implicit 
clobbers on specific ASMs.

Why use negative numbers for the hard register numbers?  I wouldn't be 
at all surprised if lots of random code assumes register numbers are 
always positive.

I don't like adding new registers with special names like !foo.  Instead 
I think that listing "!cc" or something similar in the asm itself if it 
doesn't clobber the cc register would be better.

Other opinions?


jeff