[InstCombine] Support disabling expensive combines in opt

[nikic]

Summary

Currently, there is no way to disable ExpensiveCombines when doing a standalone opt -instcombine run, as that's the default, and the opt option can currently only be used to force enable, not to force disable. The only way to disable expensive combines is via -O1 or -O2, but that of course also runs the rest of the kitchen sink...

Noticed while looking into https://bugs.llvm.org/show_bug.cgi?id=44541. Another issue I noticed is that NewPM always enables expensive combines, whiles legacy PM on does so at -O3. Anyone know whether that was an intentional choice or an oversight?

[spatel]

LGTM

For the new PM question, I assume that's an oversight. I haven't looked too much into the new PM, but I have noticed other possible mistakes there. Eg, it seems like that's running the SpeculativeExecutionPass on all targets?
https://github.com/llvm/llvm-project/blob/53b68e676faf208b4a8f817e9bd4ddd522cc6006/llvm/lib/Passes/PassBuilder.cpp#L427

vs:

https://github.com/llvm/llvm-project/blob/53b68e676faf208b4a8f817e9bd4ddd522cc6006/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp#L342

[nikic]

@spatel I've reported https://bugs.llvm.org/show_bug.cgi?id=44573 and https://bugs.llvm.org/show_bug.cgi?id=44574 to keep track of this.

[spatel]

In D72861#1824947, @nikic wrote:

@spatel I've reported https://bugs.llvm.org/show_bug.cgi?id=44573 and https://bugs.llvm.org/show_bug.cgi?id=44574 to keep track of this.

Thanks for filing those!

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

 
 static bool combineInstructionsOverFunction(
     Function &F, InstCombineWorklist &Worklist, AliasAnalysis *AA,
     AssumptionCache &AC, TargetLibraryInfo &TLI, DominatorTree &DT,
     OptimizationRemarkEmitter &ORE, BlockFrequencyInfo *BFI,
     ProfileSummaryInfo *PSI, bool ExpensiveCombines, unsigned MaxIterations,
     LoopInfo *LI) {
   auto &DL = F.getParent()->getDataLayout();
-  ExpensiveCombines |= EnableExpensiveCombines;
+  if (EnableExpensiveCombines.getNumOccurrences())
+    ExpensiveCombines = EnableExpensiveCombines;
   MaxIterations = std::min(MaxIterations, LimitMaxIterations.getValue());
 
   /// Builder - This is an IRBuilder that automatically inserts new
   /// instructions into the worklist when they are created.
   IRBuilder<TargetFolder, IRBuilderCallbackInserter> Builder(
       F.getContext(), TargetFolder(DL),
       IRBuilderCallbackInserter([&Worklist, &AC](Instruction *I) {
         Worklist.Add(I);

llvm/test/Transforms/InstCombine/expensive-combines.ll

 ;
 ; EXPENSIVE-ON-LABEL: @test(
 ; EXPENSIVE-ON-NEXT:    [[CALL:%.*]] = call i32 @passthru(i32 0)
 ; EXPENSIVE-ON-NEXT:    call void @sink(i32 0)
 ; EXPENSIVE-ON-NEXT:    ret void
 ;
 ; EXPENSIVE-OFF-LABEL: @test(
 ; EXPENSIVE-OFF-NEXT:    [[CALL:%.*]] = call i32 @passthru(i32 0)
-; EXPENSIVE-OFF-NEXT:    call void @sink(i32 0)
+; EXPENSIVE-OFF-NEXT:    call void @sink(i32 [[CALL]])
 ; EXPENSIVE-OFF-NEXT:    ret void
 ;
   %call = call i32 @passthru(i32 0)
   call void @sink(i32 %call)
   ret void
 }
 
 declare i32 @passthru(i32 returned)
 declare void @sink(i32)