[llvm] [RISCV][GISel] Fold G_FCONSTANT 0.0 store into G_CONSTANT x0 (PR #163008)

[Craig Topper via llvm-commits]

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@@ -42,6 +43,50 @@ namespace {
 #include "RISCVGenPostLegalizeGICombiner.inc"
 #undef GET_GICOMBINER_TYPES
 
+/// Match: G_STORE (G_FCONSTANT +0.0), addr
+/// Return the source vreg in MatchInfo if matched.
+bool matchFoldFPZeroStore(MachineInstr &MI, MachineRegisterInfo &MRI,
+                          Register &MatchInfo) {
+  if (MI.getOpcode() != TargetOpcode::G_STORE)
+    return false;
+
+  Register SrcReg = MI.getOperand(0).getReg();
+  if (!SrcReg.isVirtual())
+    return false;
+
+  MachineInstr *Def = MRI.getVRegDef(SrcReg);
+  if (!Def || Def->getOpcode() != TargetOpcode::G_FCONSTANT)
+    return false;
+
+  auto *CFP = Def->getOperand(1).getFPImm();
+  if (!CFP || !CFP->getValueAPF().isPosZero())
+    return false;
+
+  MatchInfo = SrcReg;
+  return true;
+}
+
+/// Apply: rewrite to G_STORE (G_CONSTANT 0 [XLEN]), addr
+void applyFoldFPZeroStore(MachineInstr &MI, MachineRegisterInfo &MRI,
+                          MachineIRBuilder &B, const RISCVSubtarget &STI,
+                          Register &MatchInfo) {
+  const unsigned XLen = STI.getXLen();
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topperc wrote:

I think we need to tell the Observer that we're changing instructions.

https://github.com/llvm/llvm-project/pull/163008