[InstCombine] Use known bits to determine exact int->fp cast

[Allen]

Summary

https://alive2.llvm.org/ce/z/GYxTRu

[Allen]

Add more cases according https://reviews.llvm.org/D125774#inline-1222628

[nikic]

It would be good to add tests for the sitofp case as well.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1759

Can use IC.computeKnownBits(Src, 0, &I) to also make use of AC/DT, I believe.

llvm/test/Transforms/InstCombine/sitofp.ll

245

According to alive, this mask would be fine: https://alive2.llvm.org/ce/z/PdpF6F This is because this is a mask of the form 0b1000... rather than 0b1111.... The right mask to test would be: https://alive2.llvm.org/ce/z/GYxTRu (We could also make use of these low zero bits, but I think that would be better as a separate change.)

[Allen]

llvm/test/Transforms/InstCombine/sitofp.ll

245

good catch. Thanks @nikic for the detailed explanations, I'll address this after this change.

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1945

We should have at least one test providing coverage for this path (and fpext too?).

IIUC, this patch will improve a case like this:

define half @masked_int_to_fp_trunc(i32 %A) {
  %m = and i32 %A, 16777215
  %B = sitofp i32 %m to float
  %C = fptrunc float %B to half
  ret half %C
}

But we don't actually need a mask:
https://alive2.llvm.org/ce/z/iaEX2i
...so the fold conditions still aren't quite right.

[Allen]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1945

hi @spatel, I think the case in your link https://alive2.llvm.org/ce/z/iaEX2i is related to another top, maybe make use of the fptrunc?

• In this patch, we hope fold the case

%m = and i25 %A, smallMaskValue
%B = uitofp i25 %m to float
%C = fptoui float %B to i25

• base on the bt we can see, it has both FPToUI and FPtoI.

#0  isKnownExactCastIntToFP (I=..., IC=...)
    at /home/zhongyunde/llvm-project-init-dev_12x/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:1762
#1  0x0000aaaaae7ee18c in llvm::InstCombinerImpl::foldItoFPtoI (this=0xffffffffbce0, FI=...)
    at /home/zhongyunde/llvm-project-init-dev_12x/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:1986
#2  0x0000aaaaae7ee398 in llvm::InstCombinerImpl::visitFPToUI (this=0xffffffffbce0, FI=...)
    at /home/zhongyunde/llvm-project-init-dev_12x/llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp:2011

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1945

I understand that we are not optimizing ideally for this pattern, but that's not my point.
These kinds of transforms are hard to get right (see for example D124692).
This patch affects 3 caller code paths, so I'd like to have tests that provide some coverage for those paths.
Please rebase with test diffs after:
a5040860412f

[Allen]

rebase

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1945

Done, thanks!

[spatel]

In D127854#3591928, @Allen wrote:

rebase

I don't think this was updated correctly. There should be test diffs from the file fpcast.ll?

[Allen]

In D127854#3592069, @spatel wrote:

In D127854#3591928, @Allen wrote:

rebase

I don't think this was updated correctly. There should be test diffs from the file fpcast.ll?

sorry, my bad. Now it is ok

[Allen]

ping ?

[Allen]

ping @spatel @nikic ?

[spatel]

LGTM - clearly, we can still do better, but this seems like a safe improvement.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1757

Please leave the TODO note. We could still check sign bits rather than zeros or do some other kind of refinement.

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

static Type *getMinimumFPType(Value *V) {
   if (Type *T = shrinkFPConstantVector(V))
     return T;
 
   return V->getType();
 }
 
 /// 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");
   Value *Src = I.getOperand(0);
   Type *SrcTy = Src->getType();
   Type *FPTy = I.getType();
   bool IsSigned = Opcode == Instruction::SIToFP;
   int SrcSize = (int)SrcTy->getScalarSizeInBits() - IsSigned;
if (match(Src, m_FPToSI(m_Value(F))) || match(Src, m_FPToUI(m_Value(F)))) {
     // [su]itofp (fpto[su]i F) --> exact if the source type has less or equal
     // significant bits than the destination (and make sure neither type is
     // weird -- ppc_fp128).
     if (SrcNumSigBits > 0 && DestNumSigBits > 0 &&
         SrcNumSigBits <= DestNumSigBits)
       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 = IC.computeKnownBits(Src, 0, &I);
+  int LowBits =
+      (int)SrcTy->getScalarSizeInBits() - SrcKnown.countMinLeadingZeros();
+  if (LowBits <= DestNumSigBits)
+    return true;
+
   return false;
 }
 
 Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
   if (Instruction *I = commonCastTransforms(FPT))
     return I;
 
   // If we have fptrunc(OpI (fpextend x), (fpextend y)), we would like to
Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
   }
 
   if (Instruction *I = shrinkInsertElt(FPT, Builder))
     return I;
 
   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);
   }
 
   return nullptr;
 }
 
 Instruction *InstCombinerImpl::visitFPExt(CastInst &FPExt) {
   // If the source operand is a cast from integer to FP and known exact, then
   // cast the integer operand directly to the destination type.
   Type *Ty = FPExt.getType();
   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);
   }
 
   return commonCastTransforms(FPExt);
 }
 
 /// fpto{s/u}i({u/s}itofp(X)) --> X or zext(X) or sext(X) or trunc(X)
 /// This is safe if the intermediate type has enough bits in its mantissa to
Instruction *InstCombinerImpl::foldItoFPtoI(CastInst &FI) {
   bool IsOutputSigned = isa<FPToSIInst>(FI);
 
   // Since we can assume the conversion won't overflow, our decision as to
   // whether the input will fit in the float should depend on the minimum
   // of the input range and output range.
 
   // 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
     // must be large enough to hold the source value exactly.
     // For example, (uint8_t)((float)(uint32_t 16777217) is undefined behavior.
     int OutputSize = (int)DestType->getScalarSizeInBits();
     if (OutputSize > OpI->getType()->getFPMantissaWidth())
       return nullptr;

llvm/test/Transforms/InstCombine/fptrunc.ll

;
   %r = fptrunc double %x to float
   ret float %r
 }
 
 ; Negative test - intermediate rounding in float type.
 
 define half @ItoFtoF_u25_f32_f16(i25 %i) {
 ; CHECK-LABEL: @ItoFtoF_u25_f32_f16(
-; CHECK-NEXT:    [[X:%.*]] = uitofp i25 [[I:%.*]] to float
-; CHECK-NEXT:    [[R:%.*]] = fptrunc float [[X]] to half
+; CHECK-NEXT:    [[R:%.*]] = uitofp i25 [[I:%.*]] to half
 ; CHECK-NEXT:    ret half [[R]]
 ;
   %x = uitofp i25 %i to float
   %r = fptrunc float %x to half
   ret half %r
 }

llvm/test/Transforms/InstCombine/sitofp.ll

 ; CHECK-NEXT:    [[C:%.*]] = fptosi double [[B]] to i55
 ; CHECK-NEXT:    ret i55 [[C]]
 ;
   %B = sitofp i64 %A to double
   %C = fptosi double %B to i55
   ret i55 %C
 }
 
-; TODO: The mask guarantees that the input is small enough to eliminate the FP casts.
+; The mask guarantees that the input is small enough to eliminate the FP casts.
 
 define i25 @masked_input(i25 %A) {
 ; CHECK-LABEL: @masked_input(
 ; CHECK-NEXT:    [[M:%.*]] = and i25 [[A:%.*]], 65535
+; CHECK-NEXT:    ret i25 [[M]]
+;
+  %m = and i25 %A, 65535
+  %B = uitofp i25 %m to float
+  %C = fptoui float %B to i25
+  ret i25 %C
+}
+
+define i25 @max_masked_input(i25 %A) {
+; CHECK-LABEL: @max_masked_input(
+; CHECK-NEXT:    [[M:%.*]] = and i25 [[A:%.*]], 16777215
+; CHECK-NEXT:    ret i25 [[M]]
+;
+  %m = and i25 %A, 16777215    ; max intermediate 16777215 (= 1 << 24)-1
+  %B = uitofp i25 %m to float
+  %C = fptoui float %B to i25
+  ret i25 %C
+}
+
+define i25 @overflow_masked_input(i25 %A) {
+; CHECK-LABEL: @overflow_masked_input(
+; CHECK-NEXT:    [[M:%.*]] = and i25 [[A:%.*]], -16777216
 ; CHECK-NEXT:    [[B:%.*]] = uitofp i25 [[M]] to float
 ; CHECK-NEXT:    [[C:%.*]] = fptoui float [[B]] to i25
 ; CHECK-NEXT:    ret i25 [[C]]
 ;
-  %m = and i25 %A, 65535
+  %m = and i25 %A, 16777216  ; Negative test - intermediate 16777216 (= 1 << 24)
   %B = uitofp i25 %m to float
   %C = fptoui float %B to i25
   ret i25 %C
 }
 
 ; TODO: Clear the low bit - guarantees that the input is converted to FP without rounding.
 
 define i25 @low_masked_input(i25 %A) {