[AMDGPU] Try to determine sign bit during div/rem expansion

[nikic]

Summary

This is preparation for D79294, which removes an expensive InstSimplify optimization, on the assumption that it will be picked up by InstCombine instead. Of course, this does not hold up if a backend performs non-trivial IR expansions without running a canonicalization pipeline afterwards, which turned up as an issue in the context of AMDGPU div/rem expansion.

This patch mitigates the issue by explicitly performing a known bits calculation where it matters. No test changes, as those would only be visible after the other patch lands.

llvm/lib/Target/AMDGPU/AMDGPUCodeGenPrepare.cpp

if (BinOpDen->getOpcode() == Instruction::Shl &&
                                0, AC, &I, DT)) {
       return true;
     }
   }
 
   return false;
 }
 
+static Value *getSign32(Value *V, IRBuilder<> &Builder, const DataLayout *DL) {
+  // Check whether the sign can be determined statically.
+  KnownBits Known = computeKnownBits(V, *DL);
+  if (Known.isNegative())
+    return Constant::getAllOnesValue(V->getType());
+  if (Known.isNonNegative())
+    return Constant::getNullValue(V->getType());
+  return Builder.CreateAShr(V, Builder.getInt32(31));
+}
+
 Value* AMDGPUCodeGenPrepare::expandDivRem32(IRBuilder<> &Builder,
                                             BinaryOperator &I,
                                             Value *Num, Value *Den) const {
   Instruction::BinaryOps Opc = I.getOpcode();
   assert(Opc == Instruction::URem || Opc == Instruction::UDiv ||
          Opc == Instruction::SRem || Opc == Instruction::SDiv);
 
   FastMathFlags FMF;
return IsSigned ? Builder.CreateSExtOrTrunc(Res, Ty) :
                       Builder.CreateZExtOrTrunc(Res, Ty);
   }
 
   ConstantInt *Zero = Builder.getInt32(0);
   ConstantInt *One = Builder.getInt32(1);
 
   Value *Sign = nullptr;
   if (IsSigned) {
-    ConstantInt *K31 = Builder.getInt32(31);
-    Value *LHSign = Builder.CreateAShr(Num, K31);
-    Value *RHSign = Builder.CreateAShr(Den, K31);
+    Value *LHSign = getSign32(Num, Builder, DL);
+    Value *RHSign = getSign32(Den, Builder, DL);
     // Remainder sign is the same as LHS
     Sign = IsDiv ? Builder.CreateXor(LHSign, RHSign) : LHSign;
 
     Num = Builder.CreateAdd(Num, LHSign);
     Den = Builder.CreateAdd(Den, RHSign);
 
     Num = Builder.CreateXor(Num, LHSign);
     Den = Builder.CreateXor(Den, RHSign);