[DCE] Don't remove non-willreturn calls

[nikic]

Summary

In both ADCE and BDCE (via DemandedBits) we should not remove instructions that are not guaranteed to return. This issue was pointed out by @fhahn in the recent llvm-dev thread.

[jdoerfert]

LGTM

llvm/lib/Analysis/DemandedBits.cpp

 }
 
 void DemandedBitsWrapperPass::print(raw_ostream &OS, const Module *M) const {
   DB->print(OS);
 }
 
 static bool isAlwaysLive(Instruction *I) {
   return I->isTerminator() || isa<DbgInfoIntrinsic>(I) || I->isEHPad() ||
-         I->mayHaveSideEffects();
+         I->mayHaveSideEffects() || !I->willReturn();
 }
 
 void DemandedBits::determineLiveOperandBits(
     const Instruction *UserI, const Value *Val, unsigned OperandNo,
     const APInt &AOut, APInt &AB, KnownBits &Known, KnownBits &Known2,
     bool &KnownBitsComputed) {
   unsigned BitWidth = AB.getBitWidth();
 

llvm/lib/Transforms/Scalar/ADCE.cpp

void AggressiveDeadCodeElimination::initialize() {
   // Build initial collection of blocks with dead terminators
   for (auto &BBInfo : BlockInfo)
     if (!BBInfo.second.terminatorIsLive())
       BlocksWithDeadTerminators.insert(BBInfo.second.BB);
 }
 
 bool AggressiveDeadCodeElimination::isAlwaysLive(Instruction &I) {
   // TODO -- use llvm::isInstructionTriviallyDead
-  if (I.isEHPad() || I.mayHaveSideEffects()) {
+  if (I.isEHPad() || I.mayHaveSideEffects() || !I.willReturn()) {
     // Skip any value profile instrumentation calls if they are
     // instrumenting constants.
     if (isInstrumentsConstant(I))
       return false;
     return true;
   }
   if (!I.isTerminator())
     return false;

llvm/test/Transforms/ADCE/dce_pure_call.ll

 ; RUN: opt -adce -S < %s | not grep call
 
-declare i32 @strlen(i8*) readonly nounwind
+declare i32 @strlen(i8*) readonly nounwind willreturn
 
 define void @test() {
   call i32 @strlen( i8* null )    ; <i32>:1 [#uses=0]
   ret void
 }

llvm/test/Transforms/ADCE/willreturn.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -adce -S < %s | FileCheck %s
 
 declare void @may_not_return(i32) nounwind readnone
 declare void @will_return(i32) nounwind readnone willreturn
 
-; FIXME: This is a miscompile.
 define void @test(i32 %a) {
 ; CHECK-LABEL: @test(
+; CHECK-NEXT:    [[B:%.*]] = add i32 [[A:%.*]], 1
+; CHECK-NEXT:    call void @may_not_return(i32 [[B]])
 ; CHECK-NEXT:    ret void
 ;
   %b = add i32 %a, 1
   call void @may_not_return(i32 %b)
   %c = add i32 %b, 1
   call void @will_return(i32 %c)
   ret void
 }

llvm/test/Transforms/BDCE/dce-pure.ll

 ; RUN: opt -bdce -S < %s | FileCheck %s
 ; RUN: opt -passes=bdce -S < %s | FileCheck %s
 
-declare i32 @strlen(i8*) readonly nounwind
+declare i32 @strlen(i8*) readonly nounwind willreturn
 
 define void @test1() {
   call i32 @strlen( i8* null )
   ret void
 
 ; CHECK-LABEL: @test1
 ; CHECK-NOT: call
 ; CHECK: ret void

llvm/test/Transforms/BDCE/dead-void-ro.ll

 ; RUN: opt -S -bdce < %s | FileCheck %s
 
 target triple = "x86_64-unknown-linux-gnu"
 
 define void @PR34211(i16* %p) {
 ; CHECK-LABEL: @PR34211
   %not_demanded_but_not_dead = load volatile i16, i16* %p
   call void @no_side_effects_so_dead(i16 %not_demanded_but_not_dead)
   ret void
 
 ; CHECK: %not_demanded_but_not_dead = load volatile i16, i16* %p
 ; CHECK-NEXT: ret void
 }
 
 declare void @no_side_effects_so_dead(i16) #0
 
-attributes #0 = { nounwind readnone }
+attributes #0 = { nounwind readnone willreturn }
 

llvm/test/Transforms/BDCE/willreturn.ll

 ; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
 ; RUN: opt -bdce -S < %s | FileCheck %s
 
 declare void @may_not_return(i32) nounwind readnone
 declare void @will_return(i32) nounwind readnone willreturn
 
-; FIXME: This is a miscompile.
 define void @test(i32 %a) {
 ; CHECK-LABEL: @test(
+; CHECK-NEXT:    [[B:%.*]] = add i32 [[A:%.*]], 1
+; CHECK-NEXT:    call void @may_not_return(i32 [[B]])
 ; CHECK-NEXT:    ret void
 ;
   %b = add i32 %a, 1
   call void @may_not_return(i32 %b)
   %c = add i32 %b, 1
   call void @will_return(i32 %c)
   ret void
 }