[InstCombine] Fold (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0)) (PR50183)

[RKSimon]

Summary

As discussed on PR50183, we already fold:

define <4 x i32>* @select0a(<4 x i32>* %a0, i64 %a1, i1 %a2, i64 %a3) {
  %gep0 = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %a1
  %gep1 = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %a3
  %sel = select i1 %a2, <4 x i32>* %gep0, <4 x i32>* %gep1
  ret <4 x i32> %sel
}
-->
define <4 x i32>* @select1a(<4 x i32>* %a0, i64 %a1, i1 %a2, i64 %a3) {
  %sel = select i1 %a2, i64 %a1, i64 %a3
  %gep = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %sel
  ret <4 x i32> %gep
}

This patch adds basic handling for the 'fallthrough' cases where the gep idx == 0 has been folded away to the base address:

define <4 x i32>* @select0(<4 x i32>* %a0, i64 %a1, i1 %a2) {
  %gep = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %a1
  %sel = select i1 %a2, <4 x i32>* %a0, <4 x i32>* %gep
  ret <4 x i32>* %sel
}
-->
define <4 x i32>* @select1(<4 x i32>* %a0, i64 %a1, i1 %a2) {
  %sel = select i1 %a2, i64 0, i64 %a1
  %gep = getelementptr inbounds <4 x i32>, <4 x i32>* %a0, i64 %sel
  ret <4 x i32>* %gep
}

[reames]

LGTM w/minor comment

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

2942

Please use early return to reduce nesting.

[lebedev.ri]

LG

llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp

Instruction *InstCombinerImpl::visitSelectInst(SelectInst &SI) {
   auto *FI = dyn_cast<Instruction>(FalseVal);
   if (TI && FI && TI->getOpcode() == FI->getOpcode())
     if (Instruction *IV = foldSelectOpOp(SI, TI, FI))
       return IV;
 
   if (Instruction *I = foldSelectExtConst(SI))
     return I;
 
+  // Fold (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0))
+  // Fold (select C, Ptr, (gep Ptr, Idx)) -> (gep Ptr, (select C, 0, Idx))
+  auto SelectGepWithBase = [&](GetElementPtrInst *Gep, Value *Base,
+                               bool Swap) -> GetElementPtrInst * {
+    Value *Ptr = Gep->getPointerOperand();
+    if (Gep->getNumOperands() != 2 || Gep->getPointerOperand() != Base ||
+        !Gep->hasOneUse())
+      return nullptr;
+    Type *ElementType = Gep->getResultElementType();
+    Value *Idx = Gep->getOperand(1);
+    Value *NewT = Idx;
+    Value *NewF = Constant::getNullValue(Idx->getType());
+    if (Swap)
+      std::swap(NewT, NewF);
+    Value *NewSI =
+        Builder.CreateSelect(CondVal, NewT, NewF, SI.getName() + ".idx", &SI);
+    return Gep->isInBounds()
+               ? GetElementPtrInst::CreateInBounds(ElementType, Ptr, {NewSI})
+               : GetElementPtrInst::Create(ElementType, Ptr, {NewSI});
+  };
+  if (auto *TrueGep = dyn_cast<GetElementPtrInst>(TrueVal))
+    if (auto *NewGep = SelectGepWithBase(TrueGep, FalseVal, false))
+      return NewGep;
+  if (auto *FalseGep = dyn_cast<GetElementPtrInst>(FalseVal))
+    if (auto *NewGep = SelectGepWithBase(FalseGep, TrueVal, true))
+      return NewGep;
+
   // See if we can fold the select into one of our operands.
   if (SelType->isIntOrIntVectorTy() || SelType->isFPOrFPVectorTy()) {
     if (Instruction *FoldI = foldSelectIntoOp(SI, TrueVal, FalseVal))
       return FoldI;
 
     Value *LHS, *RHS;
     Instruction::CastOps CastOp;
     SelectPatternResult SPR = matchSelectPattern(&SI, LHS, RHS, &CastOp);

llvm/test/Transforms/InstCombine/select-gep.ll

;
   %cmp = icmp ugt i64 %x, %y
   %select = select i1 %cmp, i32* %gep1, i32* %gep2
   ret i32* %select
 }
 
 ; PR50183
 define i32* @test2a(i32* %p, i64 %x, i64 %y) {
 ; CHECK-LABEL: @test2a(
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i32, i32* [[P:%.*]], i64 [[X:%.*]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i64 [[X]], [[Y:%.*]]
-; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[CMP]], i32* [[GEP]], i32* [[P]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i64 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[SELECT_IDX:%.*]] = select i1 [[CMP]], i64 [[X]], i64 0
+; CHECK-NEXT:    [[SELECT:%.*]] = getelementptr inbounds i32, i32* [[P:%.*]], i64 [[SELECT_IDX]]
 ; CHECK-NEXT:    ret i32* [[SELECT]]
 ;
   %gep = getelementptr inbounds i32, i32* %p, i64 %x
   %cmp = icmp ugt i64 %x, %y
   %select = select i1 %cmp, i32* %gep, i32* %p
   ret i32* %select
 }
 
 ; PR50183
 define i32* @test2b(i32* %p, i64 %x, i64 %y) {
 ; CHECK-LABEL: @test2b(
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds i32, i32* [[P:%.*]], i64 [[X:%.*]]
-; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i64 [[X]], [[Y:%.*]]
-; CHECK-NEXT:    [[SELECT:%.*]] = select i1 [[CMP]], i32* [[P]], i32* [[GEP]]
+; CHECK-NEXT:    [[CMP:%.*]] = icmp ugt i64 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    [[SELECT_IDX:%.*]] = select i1 [[CMP]], i64 0, i64 [[X]]
+; CHECK-NEXT:    [[SELECT:%.*]] = getelementptr inbounds i32, i32* [[P:%.*]], i64 [[SELECT_IDX]]
 ; CHECK-NEXT:    ret i32* [[SELECT]]
 ;
   %gep = getelementptr inbounds i32, i32* %p, i64 %x
   %cmp = icmp ugt i64 %x, %y
   %select = select i1 %cmp, i32* %p, i32* %gep
   ret i32* %select
 }