InstCombine: Restrict computeKnownBits() on all Values to OptLevel > 2

As part of r251146 InstCombine was extended to call computeKnownBits on
every value in the function to determine whether it happens to be
constant. This increases typical compiletime by 1-3% (5% in irgen+opt
time) in my measurements. On the other hand this case did not trigger
once in the whole llvm-testsuite.

This patch introduces the notion of ExpensiveCombines which are only
enabled for OptLevel > 2. I removed the check in InstructionSimplify as
that is called from various places where the OptLevel is not known but
given the rarity of the situation I think a check in InstCombine is
enough.

Differential Revision: http://reviews.llvm.org/D16835

llvm-svn: 263047

1 files changed, 5 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
index c251683d514..1909cbaea64 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
+++ b/llvm/lib/Transforms/InstCombine/InstCombineInternal.h
@@ -177,6 +177,8 @@ public:
 private:
   // Mode in which we are running the combiner.
   const bool MinimizeSize;
+  /// Enable combines that trigger rarely but are costly in compiletime.
+  const bool ExpensiveCombines;
 
   AliasAnalysis *AA;
 
@@ -195,11 +197,12 @@ private:
 
 public:
   InstCombiner(InstCombineWorklist &Worklist, BuilderTy *Builder,
-               bool MinimizeSize, AliasAnalysis *AA,
+               bool MinimizeSize, bool ExpensiveCombines, AliasAnalysis *AA,
                AssumptionCache *AC, TargetLibraryInfo *TLI,
                DominatorTree *DT, const DataLayout &DL, LoopInfo *LI)
       : Worklist(Worklist), Builder(Builder), MinimizeSize(MinimizeSize),
-        AA(AA), AC(AC), TLI(TLI), DT(DT), DL(DL), LI(LI), MadeIRChange(false) {}
+        ExpensiveCombines(ExpensiveCombines), AA(AA), AC(AC), TLI(TLI), DT(DT),
+        DL(DL), LI(LI), MadeIRChange(false) {}
 
   /// \brief Run the combiner over the entire worklist until it is empty.
   ///