Optionally enable more-aggressive FMA formation in DAGCombine

The heuristic used by DAGCombine to form FMAs checks that the FMUL has only one
use, but this is overly-conservative on some systems. Specifically, if the FMA
and the FADD have the same latency (and the FMA does not compete for resources
with the FMUL any more than the FADD does), there is no need for the
restriction, and furthermore, forming the FMA leaving the FMUL can still allow
for higher overall throughput and decreased critical-path length.

Here we add a new TLI callback, enableAggressiveFMAFusion, false by default, to
elide the hasOneUse check. This is enabled for PowerPC by default, as most
PowerPC systems will benefit.

Patch by Olivier Sallenave, thanks!

llvm-svn: 218120

1 files changed, 5 insertions, 0 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
index 6eded03313d..fd188fe37e2 100644
--- a/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
+++ b/llvm/lib/Target/PowerPC/PPCISelLowering.cpp
@@ -826,6 +826,11 @@ EVT PPCTargetLowering::getSetCCResultType(LLVMContext &, EVT VT) const {
   return VT.changeVectorElementTypeToInteger();
 }
 
+bool PPCTargetLowering::enableAggressiveFMAFusion(EVT VT) const {
+  assert(VT.isFloatingPoint() && "Non-floating-point FMA?");
+  return true;
+}
+
 //===----------------------------------------------------------------------===//
 // Node matching predicates, for use by the tblgen matching code.
 //===----------------------------------------------------------------------===//