[NFC][GVN] Put phi-translation of 'add' behind a switch

[peterwaller-arm]

Summary

The code in this #if 0 block appears to be a net benefit. Put it
behind a switch defaulting to off to support experimentation and as a
request for comment.

The codegen impact of enabling this that I'm currently persuing is that
it allows PRE to take place more frequently, particularly in loops with
second order recurrences.

Preliminary experimental data:

Across LNT on AArch64, 54 benchmarks are sped up by >1%, and 42 are
regressed by >1%, the geomean (exec_time_enabled / exec_time_disabled)
of these 96 "1% or greater significance" benchmarks is 0.991. For the
full set of 770 benchmarks it's 0.998.

There are two benchmarks which experience a >30% speedup, and the worst
slowdown is ~12%, and for every benchmark with a slowdown there is a
benckmark which is sped up by a greater factor.

[paulwalker-arm]

Sounds sensible to me to have this as a user choice rather than a compile time one. Can you add a test to show the change in behaviour we want to see when enabling this option?

[peterwaller-arm]

• Add test showing the effect of the switch.

llvm/lib/Analysis/PHITransAddr.cpp

 #include "llvm/Config/llvm-config.h"
 #include "llvm/IR/Constants.h"
 #include "llvm/IR/Dominators.h"
 #include "llvm/IR/Instructions.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;
 
+static cl::opt<bool> EnableAddPhiTranslation(
+    "gvn-add-phi-translation", cl::init(false), cl::Hidden,
+    cl::desc("Enable phi-translation of add instructions"));
+
 static bool CanPHITrans(Instruction *Inst) {
   if (isa<PHINode>(Inst) ||
       isa<GetElementPtrInst>(Inst))
     return true;
 
   if (isa<CastInst>(Inst) &&
       isSafeToSpeculativelyExecute(Inst))
     return true;
GetElementPtrInst *Result = GetElementPtrInst::Create(
         GEP->getSourceElementType(), GEPOps[0], makeArrayRef(GEPOps).slice(1),
         InVal->getName() + ".phi.trans.insert", PredBB->getTerminator());
     Result->setDebugLoc(Inst->getDebugLoc());
     Result->setIsInBounds(GEP->isInBounds());
     NewInsts.push_back(Result);
     return Result;
   }
 
-#if 0
+  // Handle add with a constant RHS.
+  if (EnableAddPhiTranslation && Inst->getOpcode() == Instruction::Add &&
+      isa<ConstantInt>(Inst->getOperand(1))) {
+
     // FIXME: This code works, but it is unclear that we actually want to insert
     // a big chain of computation in order to make a value available in a block.
     // This needs to be evaluated carefully to consider its cost trade offs.
 
-  // Handle add with a constant RHS.
-  if (Inst->getOpcode() == Instruction::Add &&
-      isa<ConstantInt>(Inst->getOperand(1))) {
     // PHI translate the LHS.
     Value *OpVal = InsertPHITranslatedSubExpr(Inst->getOperand(0),
                                               CurBB, PredBB, DT, NewInsts);
     if (OpVal == 0) return 0;
 
     BinaryOperator *Res = BinaryOperator::CreateAdd(OpVal, Inst->getOperand(1),
                                            InVal->getName()+".phi.trans.insert",
                                                     PredBB->getTerminator());
     Res->setHasNoSignedWrap(cast<BinaryOperator>(Inst)->hasNoSignedWrap());
     Res->setHasNoUnsignedWrap(cast<BinaryOperator>(Inst)->hasNoUnsignedWrap());
     NewInsts.push_back(Res);
     return Res;
   }
-#endif
 
   return nullptr;
 }

llvm/test/Transforms/GVN/PRE/phi-translate-add.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt -gvn -gvn-add-phi-translation=true -S | FileCheck %s --check-prefix=ADD-TRANS-ON
+; RUN: opt -gvn -gvn-add-phi-translation=false -S | FileCheck %s --check-prefix=ADD-TRANS-OFF
+
+; Test that phi translation is able to hoist a load whose address
+; depends on an add also being hoisted.
+define double @phi_translation_hoists_add(ptr %a, i64 %idx) {
+; ADD-TRANS-ON-LABEL: @phi_translation_hoists_add(
+; ADD-TRANS-ON-NEXT:  entry:
+; ADD-TRANS-ON-NEXT:    [[ADD_PHI_TRANS_INSERT:%.*]] = add nuw nsw i64 [[IDX:%.*]], 1
+; ADD-TRANS-ON-NEXT:    [[GEP_PHI_TRANS_INSERT:%.*]] = getelementptr inbounds double, ptr [[A:%.*]], i64 [[ADD_PHI_TRANS_INSERT]]
+; ADD-TRANS-ON-NEXT:    [[LOAD_PRE:%.*]] = load double, ptr [[GEP_PHI_TRANS_INSERT]], align 8
+; ADD-TRANS-ON-NEXT:    br label [[FOR_BODY:%.*]]
+; ADD-TRANS-ON:       for.body:
+; ADD-TRANS-ON-NEXT:    [[CMP:%.*]] = fcmp ole double [[LOAD_PRE]], 1.000000e+00
+; ADD-TRANS-ON-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[FOR_BODY]]
+; ADD-TRANS-ON:       exit:
+; ADD-TRANS-ON-NEXT:    ret double [[LOAD_PRE]]
+;
+; ADD-TRANS-OFF-LABEL: @phi_translation_hoists_add(
+; ADD-TRANS-OFF-NEXT:  entry:
+; ADD-TRANS-OFF-NEXT:    br label [[FOR_BODY:%.*]]
+; ADD-TRANS-OFF:       for.body:
+; ADD-TRANS-OFF-NEXT:    [[ADD:%.*]] = add nuw nsw i64 [[IDX:%.*]], 1
+; ADD-TRANS-OFF-NEXT:    [[GEP:%.*]] = getelementptr inbounds double, ptr [[A:%.*]], i64 [[ADD]]
+; ADD-TRANS-OFF-NEXT:    [[LOAD:%.*]] = load double, ptr [[GEP]], align 8
+; ADD-TRANS-OFF-NEXT:    [[CMP:%.*]] = fcmp ole double [[LOAD]], 1.000000e+00
+; ADD-TRANS-OFF-NEXT:    br i1 [[CMP]], label [[EXIT:%.*]], label [[FOR_BODY]]
+; ADD-TRANS-OFF:       exit:
+; ADD-TRANS-OFF-NEXT:    ret double [[LOAD]]
+;
+entry:
+  br label %for.body
+
+for.body:                                         ; preds = %for.body, %entry
+  %add = add nuw nsw i64 %idx, 1
+  %gep = getelementptr inbounds double, ptr %a, i64 %add
+  %load = load double, ptr %gep
+  %cmp = fcmp ole double %load, 1.000000e+00
+  br i1 %cmp, label %exit, label %for.body
+
+exit:
+  ret double %load
+}