[PATCH] D75807: [InstCombine] fold gep-of-select-of-constants (PR45084)

[Sanjay Patel via Phabricator via llvm-commits]

spatel created this revision.
spatel added reviewers: nikic, efriedma, rsmith.
Herald added subscribers: llvm-commits, hiraditya, mcrosier.
Herald added a project: LLVM.

As shown in PR45084:
https://bugs.llvm.org/show_bug.cgi?id=45084
...we failed to combine a gep with constant indexes with a pointer operand that is a select of constants.


https://reviews.llvm.org/D75807

Files:
  llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
  llvm/test/Transforms/InstCombine/getelementptr.ll
  llvm/test/Transforms/InstCombine/mem-gep-zidx.ll


Index: llvm/test/Transforms/InstCombine/mem-gep-zidx.ll
===================================================================
--- llvm/test/Transforms/InstCombine/mem-gep-zidx.ll
+++ llvm/test/Transforms/InstCombine/mem-gep-zidx.ll
@@ -35,9 +35,7 @@
 
 define signext i32 @test3(i32 signext %x, i1 %y) #0 {
 ; CHECK-LABEL: @test3(
-; CHECK-NEXT:    [[P:%.*]] = select i1 [[Y:%.*]], [1 x i32]* @f.a, [1 x i32]* @f.b
-; CHECK-NEXT:    [[TMP1:%.*]] = getelementptr inbounds [1 x i32], [1 x i32]* [[P]], i64 0, i64 0
-; CHECK-NEXT:    [[R:%.*]] = load i32, i32* [[TMP1]], align 4
+; CHECK-NEXT:    [[R:%.*]] = select i1 [[Y:%.*]], i32 12, i32 55
 ; CHECK-NEXT:    ret i32 [[R]]
 ;
   %idxprom = sext i32 %x to i64
Index: llvm/test/Transforms/InstCombine/getelementptr.ll
===================================================================
--- llvm/test/Transforms/InstCombine/getelementptr.ll
+++ llvm/test/Transforms/InstCombine/getelementptr.ll
@@ -1216,8 +1216,7 @@
 
 define i32* @PR45084(i1 %cond) {
 ; CHECK-LABEL: @PR45084(
-; CHECK-NEXT:    [[SEL:%.*]] = select i1 [[COND:%.*]], %struct.f* @g0, %struct.f* @g1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [[STRUCT_F:%.*]], %struct.f* [[SEL]], i64 0, i32 0
+; CHECK-NEXT:    [[GEP:%.*]] = select i1 [[COND:%.*]], i32* getelementptr inbounds (%struct.f, %struct.f* @g0, i64 0, i32 0), i32* getelementptr inbounds (%struct.f, %struct.f* @g1, i64 0, i32 0)
 ; CHECK-NEXT:    ret i32* [[GEP]]
 ;
   %sel = select i1 %cond, %struct.f* @g0, %struct.f* @g1
Index: llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -1803,6 +1803,26 @@
          (GEP2.isInBounds() || GEP2.hasAllZeroIndices());
 }
 
+/// Thread a GEP operation with constant indices through the constant true/false
+/// arms of a select.
+static Instruction *foldSelectGEP(GetElementPtrInst &GEP) {
+  if (!GEP.hasAllConstantIndices())
+    return nullptr;
+
+  Value *Cond;
+  Constant *TrueC, *FalseC;
+  if (!match(GEP.getPointerOperand(), m_Select(m_Value(Cond), m_Constant(TrueC),
+                                               m_Constant(FalseC))))
+    return nullptr;
+
+  // gep (select Cond, TrueC, FalseC), IndexC --> select Cond, TrueC', FalseC'
+  SmallVector<Value *, 4> IndexC(GEP.idx_begin(), GEP.idx_end());
+  Type *GEPSrcTy = GEP.getSourceElementType();
+  TrueC = ConstantExpr::getGetElementPtr(GEPSrcTy, TrueC, IndexC);
+  FalseC = ConstantExpr::getGetElementPtr(GEPSrcTy, FalseC, IndexC);
+  return SelectInst::Create(Cond, TrueC, FalseC);
+}
+
 Instruction *InstCombiner::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   SmallVector<Value*, 8> Ops(GEP.op_begin(), GEP.op_end());
   Type *GEPType = GEP.getType();
@@ -2446,6 +2466,9 @@
     }
   }
 
+  if (Instruction *R = foldSelectGEP(GEP))
+    return R;
+
   return nullptr;
 }
 


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