[SimplifyCFG] Tail-merging all blocks with `resume` terminator

[lebedev.ri]

Summary

Similar to what we already do for ret terminators.
As noted by @rnk, clang seems to already generate a single ret/resume,
so this isn't likely to cause widespread changes.

[rnk]

llvm/test/Transforms/SimplifyCFG/tail-merge-resume.ll

33

I thought PHIs of structs were considered non-canonical, but it looks like LLVM creates them in this C++ example:

$ cat t.cpp
struct Cleanup {
  ~Cleanup();
};
void maythrow();
void resumeFromPhi() {
  Cleanup a;
  maythrow();
  Cleanup b;
  maythrow();
}

$ clang -S t.cpp  -o - -emit-llvm -O2 | grep phi
  %.pn = phi { i8*, i32 } [ %3, %lpad1 ], [ %2, %lpad ]

We should be OK then.

91–97

I think it's worth adding an extra block where the resume has a phi operand. That is the dominant pattern coming from clang, so it's important to show that simplifycfg can take a standalone resume block and merge it in.

[lebedev.ri]

llvm/test/Transforms/SimplifyCFG/tail-merge-resume.ll

33

They are canonical, and in fact i had to invent InstCombinerImpl::foldAggregateConstructionIntoAggregateReuse()
(and i still need to finish it) so that SimplifyCFGOpt::simplifyCommonResume() would actually work...
So yes, such PHI's are absolutely fine.

[lebedev.ri]

Revisit a test somewhat.

[rnk]

lgtm, thanks

llvm/test/Transforms/SimplifyCFG/tail-merge-resume.ll

91–97

Phabricator is being weird, and this isn't highlighted as a diff, but I see that you added the semicommon block as suggested.

[lebedev.ri]

@rnk thank you for the review!

llvm/test/Transforms/SimplifyCFG/tail-merge-resume.ll

91–97

That's because i already precommitted that, and rebased the diff over the precommitted change.

llvm/lib/Transforms/Scalar/SimplifyCFGPass.cpp

if (DTU && DTU->isBBPendingDeletion(&BB))
       continue;
 
     // We are only interested in function-terminating blocks.
     if (!succ_empty(&BB))
       continue;
 
     auto *Term = BB.getTerminator();
 
-    // Fow now only support `ret` function terminators.
+    // Fow now only support `ret`/`resume` function terminators.
     // FIXME: lift this restriction.
-    if (Term->getOpcode() != Instruction::Ret)
+    switch (Term->getOpcode()) {
+    case Instruction::Ret:
+    case Instruction::Resume:
+      break;
+    default:
       continue;
+    }
 
     // We can't tail-merge block that contains a musttail call.
     if (BB.getTerminatingMustTailCall())
       continue;
 
     // Calls to experimental_deoptimize must be followed by a return
     // of the value computed by experimental_deoptimize.
     // I.e., we can not change `ret` to `br` for this block.

llvm/test/Transforms/SimplifyCFG/tail-merge-resume.ll

 ; CHECK:       invoke.cont3:
 ; CHECK-NEXT:    ret void
 ; CHECK:       lpad:
 ; CHECK-NEXT:    [[LP:%.*]] = landingpad { i8*, i32 }
 ; CHECK-NEXT:    cleanup
 ; CHECK-NEXT:    call void @foo()
 ; CHECK-NEXT:    br i1 [[COND:%.*]], label [[RESUME0:%.*]], label [[RESUME1:%.*]]
 ; CHECK:       lpad2:
 ; CHECK-NEXT:    [[LP2:%.*]] = landingpad { i8*, i32 }
 ; CHECK-NEXT:    cleanup
 ; CHECK-NEXT:    call void @bar()
 ; CHECK-NEXT:    br label [[RESUME2:%.*]]
 ; CHECK:       lpad3:
 ; CHECK-NEXT:    [[LP3:%.*]] = landingpad { i8*, i32 }
 ; CHECK-NEXT:    cleanup
 ; CHECK-NEXT:    call void @baz()
 ; CHECK-NEXT:    br label [[RESUME2]]
+; CHECK:       common.resume:
+; CHECK-NEXT:    [[COMMON_RESUME_OP:%.*]] = phi { i8*, i32 } [ [[LP]], [[RESUME0]] ], [ [[LP]], [[RESUME1]] ], [ [[SEMICOMMON_LP:%.*]], [[RESUME2]] ]
+; CHECK-NEXT:    call void @common()
+; CHECK-NEXT:    resume { i8*, i32 } [[COMMON_RESUME_OP]]
 ; CHECK:       resume0:
 ; CHECK-NEXT:    call void @qux()
-; CHECK-NEXT:    call void @common()
-; CHECK-NEXT:    resume { i8*, i32 } [[LP]]
+; CHECK-NEXT:    br label [[COMMON_RESUME:%.*]]
 ; CHECK:       resume1:
 ; CHECK-NEXT:    call void @quux()
-; CHECK-NEXT:    call void @common()
-; CHECK-NEXT:    resume { i8*, i32 } [[LP]]
+; CHECK-NEXT:    br label [[COMMON_RESUME]]
 ; CHECK:       resume2:
-; CHECK-NEXT:    [[SEMICOMMON_LP:%.*]] = phi { i8*, i32 } [ [[LP2]], [[LPAD2]] ], [ [[LP3]], [[LPAD3]] ]
+; CHECK-NEXT:    [[SEMICOMMON_LP]] = phi { i8*, i32 } [ [[LP2]], [[LPAD2]] ], [ [[LP3]], [[LPAD3]] ]
 ; CHECK-NEXT:    call void @quuz()
-; CHECK-NEXT:    call void @common()
-; CHECK-NEXT:    resume { i8*, i32 } [[SEMICOMMON_LP]]
+; CHECK-NEXT:    br label [[COMMON_RESUME]]
 ;
   invoke void @maybe_throws() to label %invoke.cont unwind label %lpad
 
 invoke.cont:
   invoke void @maybe_throws() to label %invoke.cont2 unwind label %lpad2
 
 invoke.cont2:
   invoke void @maybe_throws() to label %invoke.cont3 unwind label %lpad3

 resume0:
   call void @qux()
   call void @common()
   resume { i8*, i32 } %lp
 
 resume1:
   call void @quux()
   call void @common()
   resume { i8*, i32 } %lp
 
 resume2:
   %semicommon.lp = phi { i8*, i32 } [ %lp2, %lpad2 ], [ %lp3, %lpad3 ]
   call void @quuz()
   call void @common()
   resume { i8*, i32 } %semicommon.lp
 }
 
 declare dso_local i32 @__gxx_personality_v0(...)