> -mxop implies -mfma4, but reverse is not true.
I think this handling went in for bdver1.
But, with bdver2, we have both fma and fma4.
So for bdver2, -mxop should not be enabling one of them.
> if someone set -mfma4 together
> with -mfma on the command line, we should NOT disable selected ISA
> behind user's back
If both -mfma4 and -mfma are enabled, GCC outputs fma4 instructions.
This, I think is because fma4 instruction patterns are read before fma
instruction patterns from the ".md" files.
So, enabling both -mfma4 and -mfma is not good for bdver2.
Moreover, if user tries to use, -mfma -mno-fma4 -mxop, the order in which these
options are used becomes crucial. -mxop enables -mfma4 and by instruction
patterns fma4 instructions gets listed in the assembly file.
For the below test,
double a,b,c,d;
int fn(){
a = b + c * d ;
return a;
}
#1) Using options "-O2 -mno-fma4 -mfma -mxop" outputs fma4.
(vfmaddsd b(%rip), %xmm2, %xmm1, %xmm0)
#2) Using options "-O2 -mfma -mno-fma4 -mxop" outputs fma4.
(vfmaddsd b(%rip), %xmm2, %xmm1, %xmm0)
#3) Using options "-mxop -mno-fma4 -mfma" outpts fma.
(vfmadd132sd d(%rip), %xmm1, %xmm0)
As we see the order in which the options are used becomes crucial.
This is confusing.
I haven't really tested other implied options. But, I suspect
similar phenomenon in those cases too.
IMO, we can directly go by the CPUID flags and enable the flags.
This will be a one to one mapping and leave the user with lot more liberty.
Please let me know your opinion.
Regards
Ganesh
-----Original Message-----
From: Uros Bizjak [mailto:ubiz...@gmail.com]
Sent: Friday, August 10, 2012 1:21 AM
To: Gopalasubramanian, Ganesh
Cc: gcc-patches@gcc.gnu.org
Subject: Re: [PATCH,i386] fma,fma4 and xop flags
On Wed, Aug 8, 2012 at 1:31 PM, <ganesh.gopalasubraman...@amd.com> wrote:
> Bdver2 cpu supports both fma and fma4 instructions.
> Previous to patch, option "-mno-xop" removes "-mfma4".
> Similarly, option "-mno-fma4" removes "-mxop".
It looks to me that there is some misunderstanding. AFAICS:
-mxop implies -mfma4, but reverse is not true. Please see
#define OPTION_MASK_ISA_FMA4_SET \
(OPTION_MASK_ISA_FMA4 | OPTION_MASK_ISA_SSE4A_SET \
| OPTION_MASK_ISA_AVX_SET)
#define OPTION_MASK_ISA_XOP_SET \
(OPTION_MASK_ISA_XOP | OPTION_MASK_ISA_FMA4_SET)
So, -mxop sets -mfma4, etc ..., but -mfma4 does NOT enable -mxop.
OTOH,
#define OPTION_MASK_ISA_FMA4_UNSET \
(OPTION_MASK_ISA_FMA4 | OPTION_MASK_ISA_XOP_UNSET)
#define OPTION_MASK_ISA_XOP_UNSET OPTION_MASK_ISA_XOP
-mno-fma4 implies -mno-xop, but again reverse is not true. Thus,
-mno-xop does NOT imply -mno-fma4.
> So, the patch conditionally disables "-mfma" or "-mfma4".
> Enabling "-mxop" is done by also checking "-mfma".
Please note that conditional handling of ISA flags belongs to
ix86_option_override_internal. However, if someone set -mfma4 together
with -mfma on the command line, we should NOT disable selected ISA
behind user's back, in the same way as we don't disable anything with
"-march=i386 -msse4". With -march=bdver2, we already marked that only
fma is supported, and if user selected "-march=bdver2 -mfma4" on the
command line, we shouldn't disable anything.
Uros.
