llvm dev - Sep 2012 - [LLVMdev] Error running spec benchmark with FMA4 on X86

Hi All,

I am facing miscompare error when running povray (and few other C/C++
benchmarks) from spec cpu2006 suite enabling FMA4 (and disabling FMA3). I
have used -ffp-contract=fast to turn on this option. (Compilation options
and targets pasted below).
>>>>>>>>
clang version 3.2 (trunk 163295:163308) (llvm/trunk 163295)
Target: x86_64-unknown-linux-gnu
Thread model: posix


(Options to clang)

 -O3 -march=bdver2 -mavx -mno-fma -mfma4 -ffp-contract=fast -save-temps

<<<<<<<

Note that BDVER2 supports both FMA3 and FMA4. Also the benchmark was
run *successfully* when FMA3 was enabled. Reducing the testcase might take
more time but has anyone noticed this issue?

For those interested, miscompare message is as below

*** Miscompare of SPEC-benchmark.log; for details see
    CPU2006/peak_ref_llvm.0037/SPEC-benchmark.log.mis

0050:  Pixels:             1280   Samples:            8960
           Pixels:             1280   Samples:           47360

 ^
0051:  Rays:               8960   Saved:                 0   Max Level: 1/5
           Rays:              47360   Saved:                 0   Max Level:
1/5
                                          ^
0055:  CSG Intersection                232960               0
           Box                              47360               0
           ^
0056:  Plane                           537600          510304
          Cone/Cylinder                  1515520               0
           ^
...







-- 
* Anitha*
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On 6 September 2012 20:02, Anitha Boyapati <anitha.boyapati at
gmail.com>wrote:
>
> Hi All,
>
> I am facing miscompare error when running povray (and few other C/C++
> benchmarks) from spec cpu2006 suite enabling FMA4 (and disabling FMA3). I
> have used -ffp-contract=fast to turn on this option. (Compilation options
> and targets pasted below).
>
> >>>>>>>>
> clang version 3.2 (trunk 163295:163308) (llvm/trunk 163295)
> Target: x86_64-unknown-linux-gnu
> Thread model: posix
>
>
> (Options to clang)
>
>  -O3 -march=bdver2 -mavx -mno-fma -mfma4 -ffp-contract=fast -save-temps
>
> <<<<<<<
>
> Note that BDVER2 supports both FMA3 and FMA4. Also the benchmark was
> run *successfully* when FMA3 was enabled. Reducing the testcase might take
> more time but has anyone noticed this issue?
>

What is more interesting is that DragonEgg compiled for the same llvm
revision does well with FMA4. (Note that for Dragonegg case,
DAGCombiner.cpp is tweaked to do FMA check for AllowFpFusion() == Standard
instead of Fast. Only then FMA4 gets generated. This is just for
experimental purposes)
>

-- 
* Anitha*
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