[IndVars] Eliminate redundant type cast with different sizes

Deal with different sizes between the itofp and fptoi with
trunc or sext/zext, depend on D129756.
Fixes https://github.com/llvm/llvm-project/issues/55505.

Reviewed By: nikic

Differential Revision: https://reviews.llvm.org/D129958

1 files changed, 22 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
index 0ab79a3..0a856ee 100644
--- a/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyIndVar.cpp
@@ -683,7 +683,7 @@ bool SimplifyIndvar::replaceFloatIVWithIntegerIV(Instruction *UseInst) {
       UseInst->getOpcode() != CastInst::UIToFP)
     return false;
 
-  Value *IVOperand = UseInst->getOperand(0);
+  Instruction *IVOperand = cast<Instruction>(UseInst->getOperand(0));
   // Get the symbolic expression for this instruction.
   const SCEV *IV = SE->getSCEV(IVOperand);
   unsigned MaskBits;
@@ -696,17 +696,35 @@ bool SimplifyIndvar::replaceFloatIVWithIntegerIV(Instruction *UseInst) {
     for (User *U : UseInst->users()) {
       // Match for fptosi/fptoui of sitofp and with same type.
       auto *CI = dyn_cast<CastInst>(U);
-      if (!CI || IVOperand->getType() != CI->getType())
+      if (!CI)
         continue;
 
       CastInst::CastOps Opcode = CI->getOpcode();
       if (Opcode != CastInst::FPToSI && Opcode != CastInst::FPToUI)
         continue;
 
-      CI->replaceAllUsesWith(IVOperand);
+      Value *Conv = nullptr;
+      if (IVOperand->getType() != CI->getType()) {
+        IRBuilder<> Builder(CI);
+        StringRef Name = IVOperand->getName();
+        // To match InstCombine logic, we only need sext if both fptosi and
+        // sitofp are used. If one of them is unsigned, then we can use zext.
+        if (SE->getTypeSizeInBits(IVOperand->getType()) >
+            SE->getTypeSizeInBits(CI->getType())) {
+          Conv = Builder.CreateTrunc(IVOperand, CI->getType(), Name + ".trunc");
+        } else if (Opcode == CastInst::FPToUI ||
+                   UseInst->getOpcode() == CastInst::UIToFP) {
+          Conv = Builder.CreateZExt(IVOperand, CI->getType(), Name + ".zext");
+        } else {
+          Conv = Builder.CreateSExt(IVOperand, CI->getType(), Name + ".sext");
+        }
+      } else
+        Conv = IVOperand;
+
+      CI->replaceAllUsesWith(Conv);
       DeadInsts.push_back(CI);
       LLVM_DEBUG(dbgs() << "INDVARS: Replace IV user: " << *CI
-                        << " with: " << *IVOperand << '\n');
+                        << " with: " << *Conv << '\n');
 
       ++NumFoldedUser;
       Changed = true;