[InstCombine] Make use of low zero bits to determine exact int->fp cast

[Allen]

Summary

According the comment https://reviews.llvm.org/D127854#inline-1226805,
We could also make use of these low zero bits, https://alive2.llvm.org/ce/z/GYxTRu

[spatel]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1760–1761

Change variable name to SigBits since we're not only counting high zeros now?

llvm/test/Transforms/InstCombine/sitofp.ll

250

This test would be better with 16777218 (= 1 << 24 + 2). That would test the limits of high and low set bits, and the next test shows that we did not go past the limit.

[Allen]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1760–1761

Thanks @spatel，apply your comment

llvm/test/Transforms/InstCombine/sitofp.ll

250

Done

[spatel]

LGTM

[nikic]

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

1759

The "or compute low bit mask" part can probably be dropped now?

[xbolva00]

llvm/test/Transforms/InstCombine/sitofp.ll

256–269

Remove todo?

llvm/lib/Transforms/InstCombine/InstCombineCasts.cpp

if (match(Src, m_FPToSI(m_Value(F))) || match(Src, m_FPToUI(m_Value(F)))) {
     // 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.
+  // For example, compute number of sign bits.
   KnownBits SrcKnown = IC.computeKnownBits(Src, 0, &I);
-  int LowBits =
-      (int)SrcTy->getScalarSizeInBits() - SrcKnown.countMinLeadingZeros();
-  if (LowBits <= DestNumSigBits)
+  int SigBits = (int)SrcTy->getScalarSizeInBits() -
+                SrcKnown.countMinLeadingZeros() -
+                SrcKnown.countMinTrailingZeros();
+  if (SigBits <= DestNumSigBits)
     return true;
 
   return false;
 }
 
 Instruction *InstCombinerImpl::visitFPTrunc(FPTruncInst &FPT) {
   if (Instruction *I = commonCastTransforms(FPT))
     return I;

llvm/test/Transforms/InstCombine/sitofp.ll

 ; 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]]
+define i25 @consider_lowbits_masked_input(i25 %A) {
+; CHECK-LABEL: @consider_lowbits_masked_input(
+; CHECK-NEXT:    [[M:%.*]] = and i25 [[A:%.*]], -16777214
+; CHECK-NEXT:    ret i25 [[M]]
 ;
-  %m = and i25 %A, 16777216  ; Negative test - intermediate 16777216 (= 1 << 24)
+  %m = and i25 %A, 16777218  ; Make use of the low zero bits - intermediate 16777218 (= 1 << 24 + 2)
   %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 i32 @overflow_masked_input(i32 %A) {
+; CHECK-LABEL: @overflow_masked_input(
+; CHECK-NEXT:    [[M:%.*]] = and i32 [[A:%.*]], 16777217
+; CHECK-NEXT:    [[B:%.*]] = uitofp i32 [[M]] to float
+; CHECK-NEXT:    [[C:%.*]] = fptoui float [[B]] to i32
+; CHECK-NEXT:    ret i32 [[C]]
+;
+  %m = and i32 %A, 16777217  ; Negative test - intermediate 16777217 (= 1 << 24 + 1)
+  %B = uitofp i32 %m to float
+  %C = fptoui float %B to i32
+  ret i32 %C
+}
+
+; Clear the low bit - guarantees that the input is converted to FP without rounding.
 
 define i25 @low_masked_input(i25 %A) {
 ; CHECK-LABEL: @low_masked_input(
 ; CHECK-NEXT:    [[M:%.*]] = and i25 [[A:%.*]], -2
-; 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, -2
   %B = uitofp i25 %m to float
   %C = fptoui float %B to i25
   ret i25 %C
 }
 
 ; Output is small enough to ensure exact cast (overflow produces poison).