[llvm] Optimize fptrunc(x)>=C1 --> x>=C2 (PR #99475)

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-transforms

Author: None (kissholic)

<details>
<summary>Changes</summary>

Fix https://github.com/llvm/llvm-project/issues/85265#issue-2186848949

---
Full diff: https://github.com/llvm/llvm-project/pull/99475.diff


2 Files Affected:

- (modified) llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp (+31) 
- (added) llvm/test/Transforms/InstCombine/fold-fcmp-trunc.ll (+11) 


``````````diff
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
index abadf54a96767..2af3e92213f13 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp
@@ -22,10 +22,13 @@
 #include "llvm/Analysis/Utils/Local.h"
 #include "llvm/Analysis/VectorUtils.h"
 #include "llvm/IR/ConstantRange.h"
+#include "llvm/IR/Constants.h"
 #include "llvm/IR/DataLayout.h"
 #include "llvm/IR/InstrTypes.h"
+#include "llvm/IR/Instruction.h"
 #include "llvm/IR/IntrinsicInst.h"
 #include "llvm/IR/PatternMatch.h"
+#include "llvm/Support/Casting.h"
 #include "llvm/Support/KnownBits.h"
 #include "llvm/Transforms/InstCombine/InstCombiner.h"
 #include <bitset>
@@ -7882,6 +7885,30 @@ static Instruction *foldFCmpReciprocalAndZero(FCmpInst &I, Instruction *LHSI,
   return new FCmpInst(Pred, LHSI->getOperand(1), RHSC, "", &I);
 }
 
+// Fold trunc(x) < constant --> x < constant if possible.
+static Instruction *foldFCmpFpTrunc(FCmpInst &I, Instruction *LHSI,
+                                    Constant *RHSC) {
+  //
+  FCmpInst::Predicate Pred = I.getPredicate();
+
+  // Check that predicates are valid.
+  if ((Pred != FCmpInst::FCMP_OGT) && (Pred != FCmpInst::FCMP_OLT) &&
+      (Pred != FCmpInst::FCMP_OGE) && (Pred != FCmpInst::FCMP_OLE))
+    return nullptr;
+
+  auto *LType = LHSI->getOperand(0)->getType();
+  auto *RType = RHSC->getType();
+
+  if (!(LType->isFloatingPointTy() && RType->isFloatingPointTy() &&
+        LType->getTypeID() >= RType->getTypeID()))
+    return nullptr;
+
+  auto *ROperand = llvm::ConstantFP::get(
+      LType, dyn_cast<ConstantFP>(RHSC)->getValue().convertToDouble());
+
+  return new FCmpInst(Pred, LHSI->getOperand(0), ROperand, "", &I);
+}
+
 /// Optimize fabs(X) compared with zero.
 static Instruction *foldFabsWithFcmpZero(FCmpInst &I, InstCombinerImpl &IC) {
   Value *X;
@@ -8244,6 +8271,10 @@ Instruction *InstCombinerImpl::visitFCmpInst(FCmpInst &I) {
                   cast<LoadInst>(LHSI), GEP, GV, I))
             return Res;
       break;
+    case Instruction::FPTrunc:
+      if (Instruction *NV = foldFCmpFpTrunc(I, LHSI, RHSC))
+        return NV;
+      break;
   }
   }
 
diff --git a/llvm/test/Transforms/InstCombine/fold-fcmp-trunc.ll b/llvm/test/Transforms/InstCombine/fold-fcmp-trunc.ll
new file mode 100644
index 0000000000000..446111a60dd6c
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/fold-fcmp-trunc.ll
@@ -0,0 +1,11 @@
+; RUN: opt -passes=instcombine -S < %s | FileCheck %s
+
+
+;CHECK-LABEL: @src(
+;CHECK: %result = fcmp oge double %0, 1.000000e+02
+;CHECK-NEXT: ret i1 %result
+define i1 @src(double %0) {
+    %trunc = fptrunc double %0 to float
+    %result = fcmp oge float %trunc, 1.000000e+02
+    ret i1 %result
+}
\ No newline at end of file

``````````

</details>


https://github.com/llvm/llvm-project/pull/99475