[PowerPC/QPX] Fix the load/splat peephole with overlapping reads

If, in between the splat and the load (which does an implicit splat), there is
a read of the splat register, then that register must have another earlier
definition. In that case, we can't replace the load's destination register with
the splat's destination register.

Unfortunately, I don't have a small or non-fragile test case.

llvm-svn: 268152

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Target/PowerPC/PPCQPXLoadSplat.cpp b/llvm/lib/Target/PowerPC/PPCQPXLoadSplat.cpp
index 642b90e616d..816f05f4bf6 100644
--- a/llvm/lib/Target/PowerPC/PPCQPXLoadSplat.cpp
+++ b/llvm/lib/Target/PowerPC/PPCQPXLoadSplat.cpp
@@ -130,7 +130,15 @@ bool PPCQPXLoadSplat::runOnMachineFunction(MachineFunction &MF) {
           }
         }
 
-        if (MI->modifiesRegister(SplatReg, TRI)) {
+	// If this instruction defines the splat register, then we cannot move
+	// the previous definition above it. If it reads from the splat
+	// register, then it must already be alive from some previous
+	// definition, and if the splat register is different from the source
+	// register, then this definition must not be the load for which we're
+	// searching.
+        if (MI->modifiesRegister(SplatReg, TRI) ||
+            (SrcReg != SplatReg &&
+             MI->readsRegister(SplatReg, TRI))) {
           SI = Splats.erase(SI);
           continue;
         }