[PATCH] D105901: [InstCombine] Fold (select C, (gep Ptr, Idx), Ptr) -> (gep Ptr, (select C, Idx, 0)) (PR50183)

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon created this revision.
RKSimon added reviewers: spatel, reames, lebedev.ri.
Herald added a subscriber: hiraditya.
RKSimon requested review of this revision.
Herald added a project: LLVM.

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
  }


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D105901

Files:
  llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
  llvm/test/Transforms/InstCombine/select-gep.ll


Index: llvm/test/Transforms/InstCombine/select-gep.ll
===================================================================
--- llvm/test/Transforms/InstCombine/select-gep.ll
+++ llvm/test/Transforms/InstCombine/select-gep.ll
@@ -74,9 +74,9 @@
 ; 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
@@ -88,9 +88,9 @@
 ; 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
Index: llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
+++ llvm/lib/Transforms/InstCombine/InstCombineSelect.cpp
@@ -2933,6 +2933,34 @@
   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();
+    Type *ElementType = Gep->getResultElementType();
+    if (Gep->getNumOperands() == 2 && Gep->getPointerOperand() == Base &&
+        Gep->hasOneUse()) {
+      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});
+    }
+    return nullptr;
+  };
+  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))


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