[llvm] r254339 - [X86][FMA4] Prefer FMA4 to FMA

[Simon Pilgrim via llvm-commits]

Author: rksimon
Date: Mon Nov 30 16:22:06 2015
New Revision: 254339

URL: http://llvm.org/viewvc/llvm-project?rev=254339&view=rev
Log:
[X86][FMA4] Prefer FMA4 to FMA

We currently output FMA instructions on targets which support both FMA4 + FMA (i.e. later Bulldozer CPUS bdver2/bdver3/bdver4).

This patch flips this so FMA4 is preferred; this is for several reasons:

1 - FMA4 is non-destructive reducing the need for mov instructions.
2 - Its more straighforward to commute and fold inputs (although the recent work on FMA has reduced this difference).
3 - All supported targets have FMA4 performance equal or better to FMA - Piledriver (bdver2) in particular has half the throughput when executing FMA instructions.

Its looks like no future AMD processor lines will support FMA4 after the Bulldozer series so we're not causing problems for later CPUs.

Differential Revision: http://reviews.llvm.org/D14997

Modified:
    llvm/trunk/lib/Target/X86/X86Subtarget.h
    llvm/trunk/test/CodeGen/X86/fma-commute-x86.ll
    llvm/trunk/test/CodeGen/X86/fma_patterns.ll
    llvm/trunk/test/CodeGen/X86/fma_patterns_wide.ll

Modified: llvm/trunk/lib/Target/X86/X86Subtarget.h
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/X86/X86Subtarget.h?rev=254339&r1=254338&r2=254339&view=diff
==============================================================================
--- llvm/trunk/lib/Target/X86/X86Subtarget.h (original)
+++ llvm/trunk/lib/Target/X86/X86Subtarget.h Mon Nov 30 16:22:06 2015
@@ -354,9 +354,10 @@ public:
   bool hasXSAVEC() const { return HasXSAVEC; }
   bool hasXSAVES() const { return HasXSAVES; }
   bool hasPCLMUL() const { return HasPCLMUL; }
-  bool hasFMA() const { return HasFMA; }
-  // FIXME: Favor FMA when both are enabled. Is this the right thing to do?
-  bool hasFMA4() const { return HasFMA4 && !HasFMA; }
+  // Prefer FMA4 to FMA - its better for commutation/memory folding and
+  // has equal or better performance on all supported targets.
+  bool hasFMA() const { return HasFMA && !HasFMA4; }
+  bool hasFMA4() const { return HasFMA4; }
   bool hasXOP() const { return HasXOP; }
   bool hasTBM() const { return HasTBM; }
   bool hasMOVBE() const { return HasMOVBE; }

Modified: llvm/trunk/test/CodeGen/X86/fma-commute-x86.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/fma-commute-x86.ll?rev=254339&r1=254338&r2=254339&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/fma-commute-x86.ll (original)
+++ llvm/trunk/test/CodeGen/X86/fma-commute-x86.ll Mon Nov 30 16:22:06 2015
@@ -1,5 +1,5 @@
 ; RUN: llc < %s -mtriple=x86_64-pc-win32 -mcpu=core-avx2 | FileCheck %s
-; RUN: llc < %s -mtriple=x86_64-pc-win32 -mattr=+fma,+fma4 | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-pc-win32 -mattr=+fma | FileCheck %s
 ; RUN: llc < %s -mcpu=bdver2 -mtriple=x86_64-pc-win32 -mattr=-fma4 | FileCheck %s
 
 attributes #0 = { nounwind }

Modified: llvm/trunk/test/CodeGen/X86/fma_patterns.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/fma_patterns.ll?rev=254339&r1=254338&r2=254339&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/fma_patterns.ll (original)
+++ llvm/trunk/test/CodeGen/X86/fma_patterns.ll Mon Nov 30 16:22:06 2015
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=FMA
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=FMA
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=FMA4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4 -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=FMA4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512f,+avx512vl -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=AVX512
 

Modified: llvm/trunk/test/CodeGen/X86/fma_patterns_wide.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/X86/fma_patterns_wide.ll?rev=254339&r1=254338&r2=254339&view=diff
==============================================================================
--- llvm/trunk/test/CodeGen/X86/fma_patterns_wide.ll (original)
+++ llvm/trunk/test/CodeGen/X86/fma_patterns_wide.ll Mon Nov 30 16:22:06 2015
@@ -1,6 +1,6 @@
 ; NOTE: Assertions have been autogenerated by utils/update_llc_test_checks.py
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma -fp-contract=fast | FileCheck %s --check-prefix=FMA
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=FMA
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=FMA4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4 -fp-contract=fast | FileCheck %s --check-prefix=FMA4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx512f -fp-contract=fast | FileCheck %s --check-prefix=AVX512