[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/Feature/OperandBundles/adce.ll

 ; RUN: opt -S -adce < %s | FileCheck %s
 
 ; While it is normally okay to DCE out calls to @readonly_function and
 ; @readnone_function, we cannot do that if they're carrying operand
 ; bundles since the presence of unknown operand bundles implies
 ; arbitrary memory effects.
 
-declare void @readonly_function() readonly nounwind
-declare void @readnone_function() readnone nounwind
+declare void @readonly_function() readonly nounwind willreturn
+declare void @readnone_function() readnone nounwind willreturn
 
 define void @test0() {
 ; CHECK-LABEL: @test0(
  entry:
   call void @readonly_function() [ "tag"() ]
 ; CHECK: call void @readonly_function
   ret void
 }

llvm/test/LTO/X86/parallel.ll

 ; RUN: llvm-as -o %t.bc %s
 ; RUN: llvm-lto -exported-symbol=foo -exported-symbol=bar -j2 -o %t.o %t.bc
 ; RUN: llvm-nm %t.o.0 | FileCheck --check-prefix=CHECK0 %s
 ; RUN: llvm-nm %t.o.1 | FileCheck --check-prefix=CHECK1 %s
 
 ; FIXME: Investigate test failures on these architectures.
 ; UNSUPPORTED: mips, mipsel, aarch64, powerpc64
 
 target triple = "x86_64-unknown-linux-gnu"
 
 ; CHECK0-NOT: bar
 ; CHECK0: T foo
 ; CHECK0-NOT: bar
-define void @foo() {
+define void @foo() mustprogress {
   call void @bar()
   ret void
 }
 
 ; CHECK1-NOT: foo
 ; CHECK1: T bar
 ; CHECK1-NOT: foo
-define void @bar() {
+define void @bar() mustprogress {
   call void @foo()
   ret void
 }

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
 }

llvm/test/tools/gold/X86/parallel.ll

 ; RUN: llvm-as -o %t.bc %s
 ; RUN: rm -f %t.0.5.precodegen.bc %t.1.5.precodegen.bc %t.lto.o %t.lto.o1
 ; RUN: env LD_PRELOAD=%llvmshlibdir/LLVMgold%shlibext %gold -plugin %llvmshlibdir/LLVMgold%shlibext -u foo -u bar -plugin-opt lto-partitions=2 -plugin-opt save-temps -m elf_x86_64 -o %t %t.bc
 ; RUN: llvm-dis %t.0.5.precodegen.bc -o - | FileCheck --check-prefix=CHECK-BC0 %s
 ; RUN: llvm-dis %t.1.5.precodegen.bc -o - | FileCheck --check-prefix=CHECK-BC1 %s
 ; RUN: llvm-nm %t.lto.o | FileCheck --check-prefix=CHECK0 %s
 ; RUN: llvm-nm %t.lto.o1 | FileCheck --check-prefix=CHECK1 %s
 
 target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
 target triple = "x86_64-unknown-linux-gnu"
 
 ; CHECK-BC0: define dso_local void @foo
 ; CHECK-BC0: declare dso_local void @bar
 ; CHECK0-NOT: bar
 ; CHECK0: T foo
 ; CHECK0-NOT: bar
-define void @foo() {
+define void @foo() mustprogress {
   call void @bar()
   ret void
 }
 
 ; CHECK-BC1: declare dso_local void @foo
 ; CHECK-BC1: define dso_local void @bar
 ; CHECK1-NOT: foo
 ; CHECK1: T bar
 ; CHECK1-NOT: foo
-define void @bar() {
+define void @bar() mustprogress {
   call void @foo()
   ret void
 }