[Pipelines] Restore old DAE position in LTO pipeline

[nikic]

Summary

This is a partial revert of D128830, restoring the previous position of DeadArgElim in the fat LTO pipeline. The motivation for this is a major code size regression observed in Rust and illustrated in the PhaseOrdering test.

This is a conservative fix restoring the previous pipeline order. The real problem is that the LTO pipeline is just broken: It doesn't have a CGSCC function simplification pipeline. The inliner is just being run by itself! This wouldn't be a problem if fat LTO used a standard design where ArgPromotion and DAE are only run after functions have already been simplified by the inliner pipeline.

[aeubanks]

llvm/test/Transforms/PhaseOrdering/dae-dce.ll

2

should these unused prefixes be removed?

llvm/lib/Passes/PassBuilderPipelines.cpp

PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
 
   // Promote any localized globals to SSA registers.
   MPM.addPass(createModuleToFunctionPassAdaptor(PromotePass()));
 
   // Linking modules together can lead to duplicate global constant, only
   // keep one copy of each constant.
   MPM.addPass(ConstantMergePass());
 
+  // Remove unused arguments from functions.
+  MPM.addPass(DeadArgumentEliminationPass());
+
   // Reduce the code after globalopt and ipsccp.  Both can open up significant
   // simplification opportunities, and both can propagate functions through
   // function pointers.  When this happens, we often have to resolve varargs
   // calls, etc, so let instcombine do this.
   FunctionPassManager PeepholeFPM;
   PeepholeFPM.addPass(InstCombinePass());
   if (Level == OptimizationLevel::O3)
     PeepholeFPM.addPass(AggressiveInstCombinePass());
PassBuilder::buildLTODefaultPipeline(OptimizationLevel Level,
 
   // Garbage collect dead functions.
   MPM.addPass(GlobalDCEPass());
 
   // If we didn't decide to inline a function, check to see if we can
   // transform it to pass arguments by value instead of by reference.
   MPM.addPass(createModuleToPostOrderCGSCCPassAdaptor(ArgumentPromotionPass()));
 
-  // Remove unused arguments from functions.
-  MPM.addPass(DeadArgumentEliminationPass());
-
   FunctionPassManager FPM;
   // The IPO Passes may leave cruft around. Clean up after them.
   FPM.addPass(InstCombinePass());
   invokePeepholeEPCallbacks(FPM, Level);
 
   if (EnableConstraintElimination)
     FPM.addPass(ConstraintEliminationPass());
 

llvm/test/Other/new-pm-lto-defaults.ll

 ; CHECK-O-NEXT: Running analysis: OuterAnalysisManagerProxy
 ; CHECK-O-NEXT: Running pass: ReversePostOrderFunctionAttrsPass
 ; CHECK-O-NEXT: Running pass: GlobalSplitPass
 ; CHECK-O-NEXT: Running pass: WholeProgramDevirtPass
 ; CHECK-O1-NEXT: Running pass: LowerTypeTestsPass
 ; CHECK-O23SZ-NEXT: Running pass: GlobalOptPass
 ; CHECK-O23SZ-NEXT: Running pass: PromotePass
 ; CHECK-O23SZ-NEXT: Running pass: ConstantMergePass
+; CHECK-O23SZ-NEXT: Running pass: DeadArgumentEliminationPass
 ; CHECK-O23SZ-NEXT: Running pass: InstCombinePass
 ; CHECK-O3-NEXT: Running pass: AggressiveInstCombinePass
 ; CHECK-EP-Peephole-NEXT: Running pass: NoOpFunctionPass
 ; CHECK-O23SZ-NEXT: Running pass: ModuleInlinerWrapperPass
 ; CHECK-O23SZ-NEXT: Running analysis: InlineAdvisorAnalysis
 ; CHECK-O23SZ-NEXT: Running pass: InlinerPass
 ; CHECK-O23SZ-NEXT: Running pass: InlinerPass
 ; CHECK-O23SZ-NEXT: Invalidating analysis: InlineAdvisorAnalysis
 ; CHECK-O23SZ-NEXT: Running pass: GlobalOptPass
 ; CHECK-O23SZ-NEXT: Running pass: OpenMPOptPass
 ; CHECK-O23SZ-NEXT: Running pass: GlobalDCEPass
 ; CHECK-O23SZ-NEXT: Running pass: ArgumentPromotionPass
-; CHECK-O23SZ-NEXT: Running pass: DeadArgumentEliminationPass
 ; CHECK-O23SZ-NEXT: Running pass: InstCombinePass
 ; CHECK-EP-Peephole-NEXT: Running pass: NoOpFunctionPass
 ; CHECK-O23SZ-NEXT: Running pass: ConstraintEliminationPass
 ; CHECK-O23SZ-NEXT: Running pass: JumpThreadingPass
 ; CHECK-O23SZ-NEXT: Running analysis: LazyValueAnalysis
 ; CHECK-O23SZ-NEXT: Running pass: SROAPass on foo
 ; CHECK-O23SZ-NEXT: Running pass: TailCallElimPass on foo
 ; CHECK-O23SZ-NEXT: Running pass: PostOrderFunctionAttrsPass on (foo)

llvm/test/Transforms/PhaseOrdering/dae-dce.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -S -passes='default<O3>' < %s | FileCheck %s --check-prefixes=CHECK,DEFAULT
 ; RUN: opt -S -passes='lto<O3>' < %s | FileCheck %s --check-prefixes=CHECK,LTO
 
 declare void @llvm.trap()
 
 define void @do_trap(ptr %ptr) {
 ; CHECK-LABEL: @do_trap(
 ; CHECK-NEXT:    tail call void @llvm.trap()
 ; CHECK-NEXT:    unreachable

 ;
   %alloca = alloca ptr, align 4
   store ptr %ptr, ptr %alloca, align 4
   call void @do_trap(ptr noundef nonnull %alloca)
   unreachable
 }
 
 define internal void @dead_fn1() {
-; LTO-LABEL: @dead_fn1(
-; LTO-NEXT:    ret void
-;
   ret void
 }
 
 define internal void @dead_fn2() {
-; LTO-LABEL: @dead_fn2(
-; LTO-NEXT:    ret void
-;
   ret void
 }
 
 define void @test(i1 %c) {
-; DEFAULT-LABEL: @test(
-; DEFAULT-NEXT:    tail call fastcc void @capture_and_trap()
-; DEFAULT-NEXT:    unreachable
-;
-; LTO-LABEL: @test(
-; LTO-NEXT:    br i1 [[C:%.*]], label [[IF:%.*]], label [[ELSE:%.*]]
-; LTO:       if:
-; LTO-NEXT:    tail call fastcc void @capture_and_trap(ptr nonnull @dead_fn1)
-; LTO-NEXT:    unreachable
-; LTO:       else:
-; LTO-NEXT:    tail call fastcc void @capture_and_trap(ptr nonnull @dead_fn2)
-; LTO-NEXT:    unreachable
+; CHECK-LABEL: @test(
+; CHECK-NEXT:    tail call fastcc void @capture_and_trap()
+; CHECK-NEXT:    unreachable
 ;
   br i1 %c, label %if, label %else
 
 if:
   call void @capture_and_trap(ptr @dead_fn1)
   unreachable
 
 else:
   call void @capture_and_trap(ptr @dead_fn2)
   unreachable
 }
+;; NOTE: These prefixes are unused and the list is autogenerated. Do not add tests below this line:
+; DEFAULT: {{.*}}
+; LTO: {{.*}}