PNaCl gep expansion: avoid mul by 1 for i8 arrays.

Makes the bitcode a tiny bit smaller, and avoids generating
shift left by 0 code under fast-isel.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3343
TEST=*.ll tests
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/15861029

2 files changed, 23 insertions, 5 deletions

diff --git a/lib/Transforms/NaCl/ExpandGetElementPtr.cpp b/lib/Transforms/NaCl/ExpandGetElementPtr.cpp
index 974c3193de..1fe11293ca 100644
--- a/lib/Transforms/NaCl/ExpandGetElementPtr.cpp
+++ b/lib/Transforms/NaCl/ExpandGetElementPtr.cpp
@@ -106,11 +106,15 @@ static void ExpandGEP(GetElementPtrInst *GEP, DataLayout *DL, Type *PtrType) {
       } else {
         FlushOffset(&Ptr, &CurrentOffset, GEP, Debug, PtrType);
         Index = CastToPtrSize(Index, GEP, Debug, PtrType);
-        Instruction *Mul = BinaryOperator::Create(
-            Instruction::Mul, Index, ConstantInt::get(PtrType, ElementSize),
-            "gep_array", GEP);
-        Mul->setDebugLoc(Debug);
-        Ptr = BinaryOperator::Create(Instruction::Add, Ptr, Mul, "gep", GEP);
+        if (ElementSize != 1) {
+          Index = CopyDebug(
+              BinaryOperator::Create(Instruction::Mul, Index,
+                                     ConstantInt::get(PtrType, ElementSize),
+                                     "gep_array", GEP),
+              GEP);
+        }
+        Ptr = BinaryOperator::Create(Instruction::Add, Ptr,
+                                     Index, "gep", GEP);
         Ptr->setDebugLoc(Debug);
       }
     }
diff --git a/test/Transforms/NaCl/expand-getelementptr.ll b/test/Transforms/NaCl/expand-getelementptr.ll
index 2a089f2dc4..9f5a4bd8d2 100644
--- a/test/Transforms/NaCl/expand-getelementptr.ll
+++ b/test/Transforms/NaCl/expand-getelementptr.ll
@@ -4,6 +4,7 @@ target datalayout = "p:32:32:32"
 
 %MyStruct = type { i8, i32, i8 }
 %MyArray = type { [100 x i64] }
+%MyArrayOneByte = type { [100 x i8] }
 
 
 ; Test indexing struct field
@@ -57,6 +58,19 @@ define i64* @test_add_and_index(%MyArray* %ptr, i32 %index) {
 ; CHECK-NEXT: ret i64* %addr
 
 
+; Test that we don't multiply by 1 unnecessarily
+define i8* @test_add_and_index_one_byte(%MyArrayOneByte* %ptr, i32 %index) {
+  %addr = getelementptr %MyArrayOneByte* %ptr, i32 1, i32 0, i32 %index
+  ret i8* %addr
+}
+; CHECK: @test_add_and_index
+; CHECK-NEXT: %gep_int = ptrtoint %MyArrayOneByte* %ptr to i32
+; CHECK-NEXT: %gep = add i32 %gep_int, 100
+; CHECK-NEXT: %gep1 = add i32 %gep, %index
+; CHECK-NEXT: %addr = inttoptr i32 %gep1 to i8*
+; CHECK-NEXT: ret i8* %addr
+
+
 ; Test >32-bit array index
 define i64* @test_array_index64(%MyArray* %ptr, i64 %index) {
   %addr = getelementptr %MyArray* %ptr, i32 0, i32 0, i64 %index