[DSE] Don't remove nounwind invokes

[aheejin]

Summary

For non-mem-intrinsic and non-lifetime CallBases, the current
isRemovable function only checks if the CallBase 1. has no uses 2.
will return 3. does not throw:
https://github.com/llvm/llvm-project/blob/80fb7823367c1d105fcbc8f21b69205a0d68c859/llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp#L1017

But we should also exclude invokes even in case they don't throw,
because they are terminators and thus cannot be removed. While it
doesn't seem to make much sense for invokes to have an nounwind
target, this kind of code can be generated and is also valid bitcode.

[nikic]

LG

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

1019

I'd probably go for a CB->isTerminator() check here, rather than checking specifically for invokes (though i doubt this can happen with callbr in practice).

llvm/test/Transforms/DeadStoreElimination/nounwind-invoke.ll

2

Please always use update_test_checks.py for middle-end tests.

5

Datalayout and triple are likely not relevant.

[aheejin]

Address comments

[aheejin]

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

1019

OK. Then I'll merge the CB->isTerminator check into the bottom line here:

return CB->use_empty() && CB->willReturn() && CB->doesNotThrow();

And I don't think we need a separate comment for that, because it is kind of clear that we can't remove terminators.

llvm/test/Transforms/DeadStoreElimination/nounwind-invoke.ll

5

Removed.

[aheejin]

clang-format

[aheejin]

Also changed ptr to integer pointers because it requires an additional arguments in the opt command line.

llvm/lib/Transforms/Scalar/DeadStoreElimination.cpp

if (auto *CB = dyn_cast<CallBase>(I)) {
       if (auto *MI = dyn_cast<MemIntrinsic>(CB))
         return !MI->isVolatile();
 
       // Never remove dead lifetime intrinsics, e.g. because they are followed
       // by a free.
       if (CB->isLifetimeStartOrEnd())
         return false;
 
-      return CB->use_empty() && CB->willReturn() && CB->doesNotThrow();
+      return CB->use_empty() && CB->willReturn() && CB->doesNotThrow() &&
+             !CB->isTerminator();
     }
 
     return false;
   }
 
   /// Returns true if \p UseInst completely overwrites \p DefLoc
   /// (stored by \p DefInst).
   bool isCompleteOverwrite(const MemoryLocation &DefLoc, Instruction *DefInst,
                            Instruction *UseInst) {

llvm/test/Transforms/DeadStoreElimination/nounwind-invoke.ll

+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --prefix-filecheck-ir-name abc
+; RUN: opt -dse -S < %s | FileCheck %s
+
+; Make sure invokes are not removed as dead stores.
+define void @test_nounwind_invoke() personality i8* bitcast (i32 (...)*  @__gxx_personality_v0 to i8*) {
+; CHECK-LABEL: @test_nounwind_invoke(
+; CHECK-NEXT:  bb:
+; CHECK-NEXT:    [[TMP:%.*]] = alloca i32, align 4
+; CHECK-NEXT:    invoke void @foo(i32* [[TMP]])
+; CHECK-NEXT:    to label [[BB1:%.*]] unwind label [[BB2:%.*]]
+; CHECK:       bb1:
+; CHECK-NEXT:    call void @llvm.lifetime.end.p0i32(i64 4, i32* [[TMP]])
+; CHECK-NEXT:    ret void
+; CHECK:       bb2:
+; CHECK-NEXT:    [[ABCTMP1:%.*]] = landingpad { i8*, i32 }
+; CHECK-NEXT:    cleanup
+; CHECK-NEXT:    resume { i8*, i32 } [[ABCTMP1]]
+;
+bb:
+  %tmp = alloca i32, align 4
+  ; 'foo' is 'argmemonly', meaning it can only write to memory pointed by %tmp.
+  ; And this def is killed by 'call @llvm.lifetime.end.p0i32' in bb1 without
+  ; being used elsewhere, becoming a dead store. But we shouldn't remove this
+  ; because invokes are terminators and thus cannot be removed.
+  invoke void @foo(i32* %tmp)
+  to label %bb1 unwind label %bb2
+
+bb1:                                              ; preds = %bb
+  call void @llvm.lifetime.end.p0i32(i64 4, i32* %tmp)
+  ret void
+
+bb2:                                              ; preds = %bb
+  %tmp1 = landingpad { i8*, i32 }
+  cleanup
+  resume { i8*, i32 } %tmp1
+}
+
+; Function Attrs: argmemonly nocallback nofree nosync nounwind willreturn
+declare void @llvm.lifetime.end.p0i32(i64 immarg, i32* nocapture) #0
+; Function Attrs: argmemonly nounwind willreturn
+declare void @foo(i32*) #1
+declare i32 @__gxx_personality_v0(...)
+
+attributes #0 = { argmemonly nocallback nofree nosync nounwind willreturn }
+attributes #1 = { argmemonly nounwind willreturn }