[InstSimplify] Fold (select (icmp eq V 0) P (getelementptr P V)) -> (getelementptr P V)

[george.burgess.iv]

Summary

I'm unsure if we want to do all of the GEP simplifications here; I'm happy to factor some of the Ops.size() == 2 checks out and just do those if that's preferable.

[george.burgess.iv]

ping :)

[george.burgess.iv]

thanks!

llvm/trunk/lib/Analysis/InstructionSimplify.cpp

 static Value *SimplifyCmpInst(unsigned, Value *, Value *, const SimplifyQuery &,
                               unsigned);
 static Value *SimplifyICmpInst(unsigned Predicate, Value *LHS, Value *RHS,
                                const SimplifyQuery &Q, unsigned MaxRecurse);
 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.
 static Constant *getFalse(Type *Ty) {
   return ConstantInt::getFalse(Ty);
 }
 
 /// For a boolean type or a vector of boolean type, return true or a vector
if (MaxRecurse) {
         return SimplifyCmpInst(C->getPredicate(), RepOp, C->getOperand(1), Q,
                                MaxRecurse - 1);
       if (C->getOperand(1) == Op)
         return SimplifyCmpInst(C->getPredicate(), C->getOperand(0), RepOp, Q,
                                MaxRecurse - 1);
     }
   }
 
+  // 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
   // constant fold the instruction.
   if (Constant *CRepOp = dyn_cast<Constant>(RepOp)) {
     // Build a list of all constant operands.
     SmallVector<Constant *, 8> ConstOps;
     for (unsigned i = 0, e = I->getNumOperands(); i != e; ++i) {

llvm/trunk/test/Transforms/InstSimplify/select.ll

 ; CHECK-NEXT:    ret i8 [[SEL]]
 ;
   %notcond = icmp eq i1 %cond, false
   call void @llvm.assume(i1 %notcond)
   %sel = select 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
+}