[InstSimplify] Handle selects of GEPs with 0 offset

gburgessiv · gburgessiv · commit 6935aefdf0fd · 2017-05-01T23:12:08.000Z
In particular (since it wouldn't fit nicely in the summary): (select (icmp eq V 0) P (getelementptr P V)) -> (getelementptr P V) Differential Revision: https://reviews.llvm.org/D31435 llvm-svn: 301880
diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -62,6 +62,8 @@ static Value *SimplifyOrInst(Value *, Value *, const SimplifyQuery &, unsigned);
 static Value *SimplifyXorInst(Value *, Value *, const SimplifyQuery &, unsigned);
 static Value *SimplifyCastInst(unsigned, Value *, Type *,
                                const SimplifyQuery &, unsigned);
+static Value *SimplifyGEPInst(Type *, ArrayRef<Value *>, const SimplifyQuery &,
+                              unsigned);
 
 /// For a boolean type or a vector of boolean type, return false or a vector
 /// with every element false.
@@ -3491,6 +3493,17 @@ static const Value *SimplifyWithOpReplaced(Value *V, Value *Op, Value *RepOp,
     }
   }
 
+  // Same for GEPs.
+  if (auto *GEP = dyn_cast<GetElementPtrInst>(I)) {
+    if (MaxRecurse) {
+      SmallVector<Value *, 8> NewOps(GEP->getNumOperands());
+      transform(GEP->operands(), NewOps.begin(),
+                [&](Value *V) { return V == Op ? RepOp : V; });
+      return SimplifyGEPInst(GEP->getSourceElementType(), NewOps, Q,
+                             MaxRecurse - 1);
+    }
+  }
+
   // TODO: We could hand off more cases to instsimplify here.
 
   // If all operands are constant after substituting Op for RepOp then we can
diff --git a/llvm/test/Transforms/InstSimplify/select.ll b/llvm/test/Transforms/InstSimplify/select.ll
@@ -431,3 +431,22 @@ define i8 @do_not_assume_sel_cond(i1 %cond, i8 %x, i8 %y) {
   ret i8 %sel
 }
 
+define i32* @select_icmp_eq_0_gep_operand(i32* %base, i64 %n) {
+; CHECK-LABEL: @select_icmp_eq_0_gep_operand(
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr
+; CHECK-NEXT: ret i32* [[GEP]]
+  %cond = icmp eq i64 %n, 0
+  %gep = getelementptr i32, i32* %base, i64 %n
+  %r = select i1 %cond, i32* %base, i32* %gep
+  ret i32* %r
+}
+
+define i32* @select_icmp_ne_0_gep_operand(i32* %base, i64 %n) {
+; CHECK-LABEL: @select_icmp_ne_0_gep_operand(
+; CHECK-NEXT: [[GEP:%.*]] = getelementptr
+; CHECK-NEXT: ret i32* [[GEP]]
+  %cond = icmp ne i64 %n, 0
+  %gep = getelementptr i32, i32* %base, i64 %n
+  %r = select i1 %cond, i32* %gep, i32* %base
+  ret i32* %r
+}
