ValueTracking: Handle fptrunc in computeKnownFPClass

[arsenm]

Summary

This is the same as isKnownNeverNaN.

[kpn]

llvm/lib/Analysis/ValueTracking.cpp

4649

Isn't infinity an issue as well? If the input to be truncated is zero then we know the result can't be infinity. Then again, if the goal is to keep parity with the old implementation then do we need to worry about infinity yet?

[arsenm]

llvm/lib/Analysis/ValueTracking.cpp

4649

fptrunc(inf) => inf so there isn't really an issue as much of a missing optimization

[arsenm]

Handle infinities

[kpn]

LGTM

[kpn]

llvm/lib/Analysis/ValueTracking.cpp

4649

Wait, it's possible for a non-infinity value to be truncated to a value that is infinity in the smaller format, no? Shouldn't we just be checking that the value is in range, or just checking for zero if that's too much work?

[arsenm]

llvm/lib/Analysis/ValueTracking.cpp

4649

Yes, this is just broken. I got it right before and then corrected into wrong. isKnownNeverInfinity has a comment about this. We don't have infrastructure to check ranges

[kpn]

LGTM "for reals" this time.

[arsenm]

cb022084f0a0422e1c8af39ab092ee9a1bc6fe9e

llvm/lib/Analysis/ValueTracking.cpp

if (APFloat::semanticsMinExponent(DstTy) <
             APFloat::semanticsMaxExponent(SrcTy))
       Known.knownNot(fcSubnormal);
 
     // Sign bit of a nan isn't guaranteed.
     if (!Known.isKnownNeverNaN())
       Known.SignBit = std::nullopt;
     break;
   }
+  case Instruction::FPTrunc: {
+    if ((InterestedClasses & fcNan) == fcNone)
+      break;
+
+    KnownFPClass KnownSrc;
+    computeKnownFPClass(Op->getOperand(0), DemandedElts,
+                        InterestedClasses, KnownSrc, Depth + 1, Q, TLI);
+    if (KnownSrc.isKnownNeverNaN())
+      Known.knownNot(fcNan);
+
+    // Infinity needs a range check.
+    break;
+  }
   case Instruction::SIToFP:
   case Instruction::UIToFP: {
     // Cannot produce nan
     Known.knownNot(fcNan);
     if (Op->getOpcode() == Instruction::UIToFP)
       Known.signBitIsZero();
 
     if (InterestedClasses & fcInf) {

llvm/test/Transforms/Attributor/nofpclass-fptrunc.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 2
 ; RUN: opt -aa-pipeline=basic-aa -passes=attributor -attributor-manifest-internal -S < %s | FileCheck %s --check-prefixes=CHECK,TUNIT
 
 define float @ret_fptrunc(double %arg0) {
 ; CHECK-LABEL: define float @ret_fptrunc
 ; CHECK-SAME: (double [[ARG0:%.*]]) #[[ATTR0:[0-9]+]] {
 ; CHECK-NEXT:    [[EXT:%.*]] = fptrunc double [[ARG0]] to float
 ; CHECK-NEXT:    ret float [[EXT]]
 ;
   %ext = fptrunc double %arg0 to float
   ret float %ext
 }
 
 define float @ret_fptrunc_nonan(double nofpclass(nan) %arg0) {
-; CHECK-LABEL: define float @ret_fptrunc_nonan
+; CHECK-LABEL: define nofpclass(nan) float @ret_fptrunc_nonan
 ; CHECK-SAME: (double nofpclass(nan) [[ARG0:%.*]]) #[[ATTR0]] {
 ; CHECK-NEXT:    [[EXT:%.*]] = fptrunc double [[ARG0]] to float
 ; CHECK-NEXT:    ret float [[EXT]]
 ;
   %ext = fptrunc double %arg0 to float
   ret float %ext
 }
 

 ; CHECK-NEXT:    [[EXT:%.*]] = fptrunc double [[ARG0]] to float
 ; CHECK-NEXT:    ret float [[EXT]]
 ;
   %ext = fptrunc double %arg0 to float
   ret float %ext
 }
 
 define float @ret_fptrunc_nonan_noinf(double nofpclass(nan inf) %arg0) {
-; CHECK-LABEL: define float @ret_fptrunc_nonan_noinf
+; CHECK-LABEL: define nofpclass(nan) float @ret_fptrunc_nonan_noinf
 ; CHECK-SAME: (double nofpclass(nan inf) [[ARG0:%.*]]) #[[ATTR0]] {
 ; CHECK-NEXT:    [[EXT:%.*]] = fptrunc double [[ARG0]] to float
 ; CHECK-NEXT:    ret float [[EXT]]
 ;
   %ext = fptrunc double %arg0 to float
   ret float %ext
 }