[GISel]: Don't create G_MUL with 1 during translation of GetElementPtr

[aditya_nandakumar]

Summary

When element size is 1, it's just wasteful to create MUL with 1.

[aditya_nandakumar]

Pushed in rL321857

lib/CodeGen/GlobalISel/IRTranslator.cpp

if (StructType *StTy = GTI.getStructTypeOrNull()) {
         unsigned OffsetReg =
             getOrCreateVReg(*ConstantInt::get(OffsetIRTy, Offset));
         MIRBuilder.buildGEP(NewBaseReg, BaseReg, OffsetReg);
 
         BaseReg = NewBaseReg;
         Offset = 0;
       }
 
-      // N = N + Idx * ElementSize;
-      unsigned ElementSizeReg =
-          getOrCreateVReg(*ConstantInt::get(OffsetIRTy, ElementSize));
-
       unsigned IdxReg = getOrCreateVReg(*Idx);
       if (MRI->getType(IdxReg) != OffsetTy) {
         unsigned NewIdxReg = MRI->createGenericVirtualRegister(OffsetTy);
         MIRBuilder.buildSExtOrTrunc(NewIdxReg, IdxReg);
         IdxReg = NewIdxReg;
       }
 
-      unsigned OffsetReg = MRI->createGenericVirtualRegister(OffsetTy);
-      MIRBuilder.buildMul(OffsetReg, ElementSizeReg, IdxReg);
+      // N = N + Idx * ElementSize;
+      // Avoid doing it for ElementSize of 1.
+      unsigned GepOffsetReg;
+      if (ElementSize != 1) {
+        unsigned ElementSizeReg =
+            getOrCreateVReg(*ConstantInt::get(OffsetIRTy, ElementSize));
+
+        GepOffsetReg = MRI->createGenericVirtualRegister(OffsetTy);
+        MIRBuilder.buildMul(GepOffsetReg, ElementSizeReg, IdxReg);
+      } else
+        GepOffsetReg = IdxReg;
 
       unsigned NewBaseReg = MRI->createGenericVirtualRegister(PtrTy);
-      MIRBuilder.buildGEP(NewBaseReg, BaseReg, OffsetReg);
+      MIRBuilder.buildGEP(NewBaseReg, BaseReg, GepOffsetReg);
       BaseReg = NewBaseReg;
     }
   }
 
   if (Offset != 0) {
     unsigned OffsetReg = getOrCreateVReg(*ConstantInt::get(OffsetIRTy, Offset));
     MIRBuilder.buildGEP(getOrCreateVReg(U), BaseReg, OffsetReg);
     return true;

test/CodeGen/AArch64/GlobalISel/translate-gep.ll

 ; RUN: llc -mtriple=aarch64-linux-gnu -O0 -global-isel -stop-after=irtranslator -o - %s | FileCheck %s
 
 %type = type [4 x {i8, i32}]
 
+define i8*  @translate_element_size1(i64 %arg) {
+; CHECK-LABEL: name: translate_element_size1
+; CHECK: [[OFFSET:%[0-9]+]]:_(s64) = COPY %x0
+; CHECK: [[BASE:%[0-9]+]]:_(p0) = G_CONSTANT i64 0
+; CHECK: [[GEP:%[0-9]+]]:_(p0) = G_GEP [[BASE]], [[OFFSET]]
+  %tmp = getelementptr i8, i8* null, i64 %arg
+  ret i8* %tmp
+}
+
 define %type* @first_offset_const(%type* %addr) {
 ; CHECK-LABEL: name: first_offset_const
 ; CHECK: [[BASE:%[0-9]+]]:_(p0) = COPY %x0
 ; CHECK: [[OFFSET:%[0-9]+]]:_(s64) = G_CONSTANT i64 32
 ; CHECK: [[RES:%[0-9]+]]:_(p0) = G_GEP [[BASE]], [[OFFSET]](s64)
 ; CHECK: %x0 = COPY [[RES]](p0)
 
   %res = getelementptr %type, %type* %addr, i32 1