[InstCombine] Canonicalize more geps with constant gep bases and constant offsets.

This is another small but hopefully not performance negative step to
canonicalizing towards i8 geps. We looks for geps with a constant offset base
pointer of the form `gep (gep @glob, C1), x, C2` and expand the gep
instruction, so that the constant can hopefully be combined together (or the
offset can be computed in common).

Author: davemgreen

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2822,11 +2822,17 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
   //    This has better support in BasicAA.
   //  - gep i32 p, mul(O, C) -> gep i8, p, mul(O, C*4) to fold the two
   //    multiplies together.
-  if (GEPEltType->isScalableTy() ||
-      (!GEPEltType->isIntegerTy(8) && GEP.getNumIndices() == 1 &&
-       match(GEP.getOperand(1),
-             m_OneUse(m_CombineOr(m_Mul(m_Value(), m_ConstantInt()),
-                                  m_Shl(m_Value(), m_ConstantInt())))))) {
+  //  - gep (gep @global, C1), %x, C2 is expanded so the two constants can
+  //    possibly be merged together.
+  if (!GEPEltType->isIntegerTy(8) &&
+      (GEPEltType->isScalableTy() ||
+       (GEP.getNumIndices() == 1 &&
+        match(GEP.getOperand(1),
+              m_OneUse(m_CombineOr(m_Mul(m_Value(), m_ConstantInt()),
+                                   m_Shl(m_Value(), m_ConstantInt()))))) ||
+       (isa<GEPOperator>(PtrOp) && isa<ConstantExpr>(PtrOp) &&
+        any_of(drop_begin(GEP.indices()),
+               [](Value *V) { return isa<Constant>(V); })))) {
     Value *Offset = EmitGEPOffset(cast<GEPOperator>(&GEP));
     return replaceInstUsesWith(
         GEP, Builder.CreatePtrAdd(PtrOp, Offset, "", GEP.getNoWrapFlags()));

llvm/test/Transforms/InstCombine/canonicalize-gep-constglob.ll

@@ -7,7 +7,8 @@ define ptr @x12(i64 %x) {
 ; CHECK-LABEL: define ptr @x12(
 ; CHECK-SAME: i64 [[X:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [10 x [10 x [10 x i32]]], ptr getelementptr inbounds (i8, ptr @glob, i64 36), i64 0, i64 [[X]], i64 1, i64 2
+; CHECK-NEXT:    [[GEP_IDX:%.*]] = mul nsw i64 [[X]], 400
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr getelementptr inbounds (i8, ptr @glob, i64 84), i64 [[GEP_IDX]]
 ; CHECK-NEXT:    ret ptr [[GEP]]
 ;
 entry:
@@ -19,7 +20,10 @@ define ptr @x1y(i64 %x, i64 %y) {
 ; CHECK-LABEL: define ptr @x1y(
 ; CHECK-SAME: i64 [[X:%.*]], i64 [[Y:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [10 x [10 x [10 x i32]]], ptr getelementptr inbounds (i8, ptr @glob, i64 36), i64 0, i64 [[X]], i64 2, i64 [[Y]]
+; CHECK-NEXT:    [[GEP_IDX:%.*]] = mul nsw i64 [[X]], 400
+; CHECK-NEXT:    [[GEP_IDX1:%.*]] = shl nsw i64 [[Y]], 2
+; CHECK-NEXT:    [[TMP0:%.*]] = getelementptr i8, ptr getelementptr inbounds (i8, ptr @glob, i64 116), i64 [[GEP_IDX]]
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i8, ptr [[TMP0]], i64 [[GEP_IDX1]]
 ; CHECK-NEXT:    ret ptr [[GEP]]
 ;
 entry:
@@ -43,8 +47,10 @@ define i32 @twoloads(i64 %x) {
 ; CHECK-LABEL: define i32 @twoloads(
 ; CHECK-SAME: i64 [[X:%.*]]) {
 ; CHECK-NEXT:  entry:
-; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr inbounds [10 x [10 x [10 x i32]]], ptr getelementptr inbounds (i8, ptr @glob, i64 50), i64 0, i64 [[X]], i64 2, i64 1
-; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr inbounds [10 x [10 x [10 x i32]]], ptr getelementptr inbounds (i8, ptr @glob, i64 36), i64 0, i64 [[X]], i64 2, i64 4
+; CHECK-NEXT:    [[GEP1_IDX:%.*]] = mul nsw i64 [[X]], 400
+; CHECK-NEXT:    [[GEP1:%.*]] = getelementptr i8, ptr getelementptr inbounds (i8, ptr @glob, i64 134), i64 [[GEP1_IDX]]
+; CHECK-NEXT:    [[GEP2_IDX:%.*]] = mul nsw i64 [[X]], 400
+; CHECK-NEXT:    [[GEP2:%.*]] = getelementptr i8, ptr getelementptr inbounds (i8, ptr @glob, i64 132), i64 [[GEP2_IDX]]
 ; CHECK-NEXT:    [[A:%.*]] = load i32, ptr [[GEP1]], align 4
 ; CHECK-NEXT:    [[B:%.*]] = load i32, ptr [[GEP2]], align 4
 ; CHECK-NEXT:    [[C:%.*]] = add i32 [[A]], [[B]]