[PATCH] D14997: [X86][FMA4] Prefer FMA4 to FMA

[Simon Pilgrim via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: craig.topper, spatel.
RKSimon added a subscriber: llvm-commits.
RKSimon set the repository for this revision to rL LLVM.

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. 

(Craig - it looks like it only took 3 years to make this change after your original patch at r162454!)

Repository:
  rL LLVM

http://reviews.llvm.org/D14997

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

Index: test/CodeGen/X86/fma_patterns_wide.ll
===================================================================
--- test/CodeGen/X86/fma_patterns_wide.ll
+++ test/CodeGen/X86/fma_patterns_wide.ll
@@ -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
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=CHECK_FMA4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4 -fp-contract=fast | FileCheck %s --check-prefix=CHECK_FMA4
 
 define <16 x float> @test_x86_fmadd_ps_y_wide(<16 x float> %a0, <16 x float> %a1, <16 x float> %a2) {
Index: test/CodeGen/X86/fma_patterns.ll
===================================================================
--- test/CodeGen/X86/fma_patterns.ll
+++ test/CodeGen/X86/fma_patterns.ll
@@ -1,5 +1,5 @@
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=CHECK_FMA
-; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=CHECK_FMA
+; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4,+fma -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=CHECK_FMA4
 ; RUN: llc < %s -mtriple=x86_64-unknown-unknown -mattr=+avx,+fma4 -fp-contract=fast | FileCheck %s --check-prefix=ALL --check-prefix=CHECK_FMA4
 
 ;
Index: test/CodeGen/X86/fma-commute-x86.ll
===================================================================
--- test/CodeGen/X86/fma-commute-x86.ll
+++ test/CodeGen/X86/fma-commute-x86.ll
@@ -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 }
Index: lib/Target/X86/X86Subtarget.h
===================================================================
--- lib/Target/X86/X86Subtarget.h
+++ lib/Target/X86/X86Subtarget.h
@@ -354,9 +354,10 @@
   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; }


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