[llvm] Re apply 130577 narrow math for and operand (PR #133896)

[via llvm-commits]

================
@@ -1559,6 +1559,74 @@ void AMDGPUCodeGenPrepareImpl::expandDivRem64(BinaryOperator &I) const {
   llvm_unreachable("not a division");
 }
 
+/*
+This will cause non-byte load in consistency, for example:
+```
+    %load = load i1, ptr addrspace(4) %arg, align 4
+    %zext = zext i1 %load to
+    i64 %add = add i64 %zext
+```
+Instead of creating `s_and_b32 s0, s0, 1`,
+it will create `s_and_b32 s0, s0, 0xff`.
+We accept this change since the non-byte load assumes the upper bits
+within the byte are all 0.
+*/
+static bool tryNarrowMathIfNoOverflow(Instruction *I,
+                                      const SITargetLowering *TLI,
+                                      const TargetTransformInfo &TTI,
+                                      const DataLayout &DL) {
+  unsigned Opc = I->getOpcode();
+  Type *OldType = I->getType();
+
+  if (Opc != Instruction::Add && Opc != Instruction::Mul)
+    return false;
+
+  unsigned OrigBit = OldType->getScalarSizeInBits();
+
+  if (Opc != Instruction::Add && Opc != Instruction::Mul)
+    llvm_unreachable("Unexpected opcode, only valid for Instruction::Add and "
+                     "Instruction::Mul.");
+
+  unsigned MaxBitsNeeded = computeKnownBits(I, DL).countMaxActiveBits();
+
+  MaxBitsNeeded = std::max<unsigned>(bit_ceil(MaxBitsNeeded), 8);
+  Type *NewType = DL.getSmallestLegalIntType(I->getContext(), MaxBitsNeeded);
+  if (!NewType)
+    return false;
+  unsigned NewBit = NewType->getIntegerBitWidth();
+  if (NewBit >= OrigBit)
+    return false;
+  NewType = I->getType()->getWithNewBitWidth(NewBit);
+
+  // Old cost
+  InstructionCost OldCost =
+      TTI.getArithmeticInstrCost(Opc, OldType, TTI::TCK_RecipThroughput);
+  // New cost of new op
+  InstructionCost NewCost =
+      TTI.getArithmeticInstrCost(Opc, NewType, TTI::TCK_RecipThroughput);
+  // New cost of narrowing 2 operands (use trunc)
+  NewCost += 2 * TTI.getCastInstrCost(Instruction::Trunc, NewType, OldType,
----------------
Shoreshen wrote:

Hi @jmmartinez , yep if one of them is constant, we actually doesn't need to narrow it. But based on my findings till now, the trunc in AMDGPU backend is mostly free......

https://github.com/llvm/llvm-project/pull/133896