[PowerPC] Add a late MI-level pass for QPX load/splat simplification

Chapter 3 of the QPX manual states that, "Scalar floating-point load
instructions, defined in the Power ISA, cause a replication of the source data
across all elements of the target register." Thus, if we have a load followed
by a QPX splat (from the first lane), the splat is redundant. This adds a late
MI-level pass to remove the redundant splats in some of these cases
(specifically when both occur in the same basic block).

This optimization is scheduled just prior to post-RA scheduling. It can't happen
before anything that might replace the load with some already-computed quantity
(i.e. store-to-load forwarding).

llvm-svn: 265047

1 files changed, 12 insertions, 1 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp b/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp
index 8d5af9458be..5d47e0e3ebd 100644
--- a/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp
+++ b/llvm/lib/Target/PowerPC/PPCTargetMachine.cpp
@@ -43,6 +43,10 @@ opt<bool> DisableVSXSwapRemoval("disable-ppc-vsx-swap-removal", cl::Hidden,
                                 cl::desc("Disable VSX Swap Removal for PPC"));
 
 static cl::
+opt<bool> DisableQPXLoadSplat("disable-ppc-qpx-load-splat", cl::Hidden,
+                              cl::desc("Disable QPX load splat simplification"));
+
+static cl::
 opt<bool> DisableMIPeephole("disable-ppc-peephole", cl::Hidden,
                             cl::desc("Disable machine peepholes for PPC"));
 
@@ -388,8 +392,15 @@ void PPCPassConfig::addPreRegAlloc() {
 }
 
 void PPCPassConfig::addPreSched2() {
-  if (getOptLevel() != CodeGenOpt::None)
+  if (getOptLevel() != CodeGenOpt::None) {
     addPass(&IfConverterID);
+
+    // This optimization must happen after anything that might do store-to-load
+    // forwarding. Here we're after RA (and, thus, when spills are inserted)
+    // but before post-RA scheduling.
+    if (!DisableQPXLoadSplat)
+      addPass(createPPCQPXLoadSplatPass());
+  }
 }
 
 void PPCPassConfig::addPreEmitPass() {