[ConstraintElimination] Decompose GEP %ptr, ZEXT(SHL()).

Add support to decompose a GEP with a ZEXT(SHL()) operand.

Author: fhahn

llvm/lib/Transforms/Scalar/ConstraintElimination.cpp

@@ -64,6 +64,11 @@ static SmallVector<std::pair<int64_t, Value *>, 4> decompose(Value *V) {
       return {{0, nullptr},
               {1, GEP->getPointerOperand()},
               {pow(2, CI->getSExtValue()), Op0}};
+    if (match(GEP->getOperand(GEP->getNumOperands() - 1),
+              m_ZExt(m_NUWShl(m_Value(Op0), m_ConstantInt(CI)))))
+      return {{0, nullptr},
+              {1, GEP->getPointerOperand()},
+              {pow(2, CI->getSExtValue()), Op0}};
 
     return {{0, nullptr},
             {1, GEP->getPointerOperand()},

llvm/test/Transforms/ConstraintElimination/geps.ll

@@ -455,7 +455,7 @@ define void @test.ult.gep.shl.zext(i32* readonly %src, i32* readnone %max, i32 %
 ; CHECK-NEXT:    [[EXT_1:%.*]] = zext i32 [[IDX_SHL]] to i64
 ; CHECK-NEXT:    [[ADD_PTR_SHL:%.*]] = getelementptr inbounds i32, i32* [[SRC]], i64 [[EXT_1]]
 ; CHECK-NEXT:    [[C_MAX_0:%.*]] = icmp ult i32* [[ADD_PTR_SHL]], [[MAX]]
-; CHECK-NEXT:    call void @use(i1 [[C_MAX_0]])
+; CHECK-NEXT:    call void @use(i1 true)
 ; CHECK-NEXT:    [[IDX_SHL_NOT_NUW:%.*]] = shl i32 [[IDX]], 1
 ; CHECK-NEXT:    [[EXT_2:%.*]] = zext i32 [[IDX_SHL_NOT_NUW]] to i64
 ; CHECK-NEXT:    [[ADD_PTR_SHL_NOT_NUW:%.*]] = getelementptr inbounds i32, i32* [[SRC]], i64 [[EXT_2]]