[llvm] r325630 - [MemoryBuiltins] Check nobuiltin status when identifying calls to free.

[Sam Clegg via llvm-commits]

On Wed, Feb 21, 2018 at 2:46 AM, Benjamin Kramer <benny.kra at gmail.com> wrote:
> I'm relatively certain that the test is bad wrt the C++ standard. The
> compiler is allowed to elide new even if it is user-defined. The standard's
> escape hatch is calling "::operator new(size)" instead of using a new
> expression. Building with -fno-builtin should also work. Or feed the pointer
> into an __asm__ barrier to prevent the compiler from removing it.
>

Interesting.  Why is it allowed to elide the call to new? I would have
thought that since the result of the new is used (its printed) the
call can't be elided.  But I don't know the spec so if you are sure
this is allowed I can simply disable the tests that use this pattern.

Would you have expected the change in question to have caused this to
start failing?


>
> On Wed, Feb 21, 2018 at 2:49 AM Sam Clegg <sbc at google.com> wrote:
>>
>> We have some test code on the WebAssembly waterfall the broke as a
>> result of this.  Perhaps the test is bad but I've distilled to the
>> following:
>>
>> ```
>> #include <stdio.h>
>> #include <stdlib.h>
>>
>> void* operator new(size_t s) {
>>   static int n = 1;
>>   printf("new %zu\n", s);
>>   return &n;
>> }
>>
>> int main() {
>>   int* foo = new int;
>>   printf("%p\n", foo);
>>   return 0;
>> }
>> ```
>>
>> Before this change the above program will always print "new ...".
>> After this change it only prints this O0.  In O1 and O2 the call to
>> new is elided.  Is this intended?
>>
>> On Tue, Feb 20, 2018 at 2:00 PM, Benjamin Kramer via llvm-commits
>> <llvm-commits at lists.llvm.org> wrote:
>> > Author: d0k
>> > Date: Tue Feb 20 14:00:33 2018
>> > New Revision: 325630
>> >
>> > URL: http://llvm.org/viewvc/llvm-project?rev=325630&view=rev
>> > Log:
>> > [MemoryBuiltins] Check nobuiltin status when identifying calls to free.
>> >
>> > This is usually not a problem because this code's main purpose is
>> > eliminating unused new/delete pairs. We got deletes of nullptr or
>> > nobuiltin deletes of builtin new wrong though.
>> >
>> > Modified:
>> >     llvm/trunk/lib/Analysis/MemoryBuiltins.cpp
>> >     llvm/trunk/test/Transforms/InstCombine/malloc-free-delete.ll
>> >
>> > Modified: llvm/trunk/lib/Analysis/MemoryBuiltins.cpp
>> > URL:
>> > http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/MemoryBuiltins.cpp?rev=325630&r1=325629&r2=325630&view=diff
>> >
>> > ==============================================================================
>> > --- llvm/trunk/lib/Analysis/MemoryBuiltins.cpp (original)
>> > +++ llvm/trunk/lib/Analysis/MemoryBuiltins.cpp Tue Feb 20 14:00:33 2018
>> > @@ -112,10 +112,9 @@ static const Function *getCalledFunction
>> >
>> >    IsNoBuiltin = CS.isNoBuiltin();
>> >
>> > -  const Function *Callee = CS.getCalledFunction();
>> > -  if (!Callee || !Callee->isDeclaration())
>> > -    return nullptr;
>> > -  return Callee;
>> > +  if (const Function *Callee = CS.getCalledFunction())
>> > +    return Callee;
>> > +  return nullptr;
>> >  }
>> >
>> >  /// Returns the allocation data for the given value if it's either a
>> > call to a
>> > @@ -350,11 +349,10 @@ const CallInst *llvm::extractCallocCall(
>> >
>> >  /// isFreeCall - Returns non-null if the value is a call to the builtin
>> > free()
>> >  const CallInst *llvm::isFreeCall(const Value *I, const
>> > TargetLibraryInfo *TLI) {
>> > -  const CallInst *CI = dyn_cast<CallInst>(I);
>> > -  if (!CI || isa<IntrinsicInst>(CI))
>> > -    return nullptr;
>> > -  Function *Callee = CI->getCalledFunction();
>> > -  if (Callee == nullptr)
>> > +  bool IsNoBuiltinCall;
>> > +  const Function *Callee =
>> > +      getCalledFunction(I, /*LookThroughBitCast=*/false,
>> > IsNoBuiltinCall);
>> > +  if (Callee == nullptr || IsNoBuiltinCall)
>> >      return nullptr;
>> >
>> >    StringRef FnName = Callee->getName();
>> > @@ -400,7 +398,7 @@ const CallInst *llvm::isFreeCall(const V
>> >    if (FTy->getParamType(0) != Type::getInt8PtrTy(Callee->getContext()))
>> >      return nullptr;
>> >
>> > -  return CI;
>> > +  return dyn_cast<CallInst>(I);
>> >  }
>> >
>> >
>> > //===----------------------------------------------------------------------===//
>> >
>> > Modified: llvm/trunk/test/Transforms/InstCombine/malloc-free-delete.ll
>> > URL:
>> > http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/malloc-free-delete.ll?rev=325630&r1=325629&r2=325630&view=diff
>> >
>> > ==============================================================================
>> > --- llvm/trunk/test/Transforms/InstCombine/malloc-free-delete.ll
>> > (original)
>> > +++ llvm/trunk/test/Transforms/InstCombine/malloc-free-delete.ll Tue Feb
>> > 20 14:00:33 2018
>> > @@ -146,7 +146,11 @@ lpad.i:
>> >  }
>> >
>> >  declare i8* @_Znwm(i64) nobuiltin
>> > -declare i8* @_Znwj(i32) nobuiltin
>> > +define i8* @_Znwj(i32 %n) nobuiltin {
>> > +  %z = zext i32 %n to i64
>> > +  call i8* @_Znwm(i64 %z)
>> > +  ret i8* %m
>> > +}
>> >  declare i8* @_Znam(i64) nobuiltin
>> >  declare i8* @_Znaj(i32) nobuiltin
>> >  declare void @_ZdlPv(i8*) nobuiltin
>> > @@ -197,3 +201,19 @@ define void @test9() {
>> >    call void @"\01??3 at YAXPEAX@Z"(i8* %new_long_long) builtin
>> >    ret void
>> >  }
>> > +
>> > +define void @test10()  {
>> > +; CHECK-LABEL: @test10
>> > +; CHECK: call void @_ZdlPv
>> > +  call void @_ZdlPv(i8* null)
>> > +  ret void
>> > +}
>> > +
>> > +define void @test11() {
>> > +; CHECK-LABEL: @test11
>> > +; CHECK: call i8* @_Znwm
>> > +; CHECK: call void @_ZdlPv
>> > +  %call = call i8* @_Znwm(i64 8) builtin
>> > +  call void @_ZdlPv(i8* %call)
>> > +  ret void
>> > +}
>> >
>> >
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