[SimplifyCFG] Create attribute for fuzzing-specific optimizations.

Summary:
When building with libFuzzer, converting control flow to selects or
obscuring the original operands of CMPs reduces the effectiveness of
libFuzzer's heuristics.

This patch provides an attribute to disable or modify certain optimizations
for optimal fuzzing signal.

Provides a less aggressive alternative to https://reviews.llvm.org/D44057.

Reviewers: vitalybuka, davide, arsenm, hfinkel

Reviewed By: vitalybuka

Subscribers: junbuml, mehdi_amini, wdng, javed.absar, hiraditya, llvm-commits, kcc

Differential Revision: https://reviews.llvm.org/D44232

llvm-svn: 328214

1 files changed, 7 insertions, 0 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 685fdb7ec63..5dc9d2c3b71 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2273,6 +2273,10 @@ static bool FoldTwoEntryPHINode(PHINode *PN, const TargetTransformInfo &TTI,
   // dependence information for this check, but simplifycfg can't keep it up
   // to date, and this catches most of the cases we care about anyway.
   BasicBlock *BB = PN->getParent();
+  const Function *Fn = BB->getParent();
+  if (Fn && Fn->hasFnAttribute(Attribute::OptForFuzzing))
+    return false;
+
   BasicBlock *IfTrue, *IfFalse;
   Value *IfCond = GetIfCondition(BB, IfTrue, IfFalse);
   if (!IfCond ||
@@ -5799,6 +5803,9 @@ static BasicBlock *allPredecessorsComeFromSameSource(BasicBlock *BB) {
 
 bool SimplifyCFGOpt::SimplifyCondBranch(BranchInst *BI, IRBuilder<> &Builder) {
   BasicBlock *BB = BI->getParent();
+  const Function *Fn = BB->getParent();
+  if (Fn && Fn->hasFnAttribute(Attribute::OptForFuzzing))
+    return false;
 
   // Conditional branch
   if (isValueEqualityComparison(BI)) {