[PATCH] D127854: [InstCombne] Find if the source integer value has less significant bits

[Allen zhong via Phabricator via llvm-commits]

Allen created this revision.
Allen added reviewers: nikic, spatel.
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This patch is part of D125774 <https://reviews.llvm.org/D125774>, need be commited separately.


https://reviews.llvm.org/D127854

Files:
  llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
  llvm/test/Transforms/InstCombine/sitofp.ll


Index: llvm/test/Transforms/InstCombine/sitofp.ll
===================================================================
--- llvm/test/Transforms/InstCombine/sitofp.ll
+++ llvm/test/Transforms/InstCombine/sitofp.ll
@@ -223,9 +223,7 @@
 define i25 @masked_input(i25 %A) {
 ; CHECK-LABEL: @masked_input(
 ; CHECK-NEXT:    [[M:%.*]] = and i25 [[A:%.*]], 65535
-; CHECK-NEXT:    [[B:%.*]] = uitofp i25 [[M]] to float
-; CHECK-NEXT:    [[C:%.*]] = fptoui float [[B]] to i25
-; CHECK-NEXT:    ret i25 [[C]]
+; CHECK-NEXT:    ret i25 [[M]]
 ;
   %m = and i25 %A, 65535
   %B = uitofp i25 %m to float
Index: llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp
@@ -1720,7 +1720,7 @@
 
 /// Return true if the cast from integer to FP can be proven to be exact for all
 /// possible inputs (the conversion does not lose any precision).
-static bool isKnownExactCastIntToFP(CastInst &I) {
+static bool isKnownExactCastIntToFP(CastInst &I, InstCombinerImpl &IC) {
   CastInst::CastOps Opcode = I.getOpcode();
   assert((Opcode == CastInst::SIToFP || Opcode == CastInst::UIToFP) &&
          "Unexpected cast");
@@ -1754,9 +1754,14 @@
       return true;
   }
 
-  // TODO:
   // Try harder to find if the source integer type has less significant bits.
   // For example, compute number of sign bits or compute low bit mask.
+  KnownBits SrcKnown = llvm::computeKnownBits(Src, IC.getDataLayout());
+  int LowBits =
+      (int)SrcTy->getScalarSizeInBits() - SrcKnown.countMinLeadingZeros();
+  if (LowBits <= DestNumSigBits)
+    return true;
+
   return false;
 }
 
@@ -1937,7 +1942,7 @@
   Value *Src = FPT.getOperand(0);
   if (isa<SIToFPInst>(Src) || isa<UIToFPInst>(Src)) {
     auto *FPCast = cast<CastInst>(Src);
-    if (isKnownExactCastIntToFP(*FPCast))
+    if (isKnownExactCastIntToFP(*FPCast, *this))
       return CastInst::Create(FPCast->getOpcode(), FPCast->getOperand(0), Ty);
   }
 
@@ -1951,7 +1956,7 @@
   Value *Src = FPExt.getOperand(0);
   if (isa<SIToFPInst>(Src) || isa<UIToFPInst>(Src)) {
     auto *FPCast = cast<CastInst>(Src);
-    if (isKnownExactCastIntToFP(*FPCast))
+    if (isKnownExactCastIntToFP(*FPCast, *this))
       return CastInst::Create(FPCast->getOpcode(), FPCast->getOperand(0), Ty);
   }
 
@@ -1978,7 +1983,7 @@
 
   // This means this is also safe for a signed input and unsigned output, since
   // a negative input would lead to undefined behavior.
-  if (!isKnownExactCastIntToFP(*OpI)) {
+  if (!isKnownExactCastIntToFP(*OpI, *this)) {
     // The first cast may not round exactly based on the source integer width
     // and FP width, but the overflow UB rules can still allow this to fold.
     // If the destination type is narrow, that means the intermediate FP value


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