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

Wed Feb 21 12:44:20 PST 2018

Interestingly it doesn't elide the call if I return a non-const value.
If I write `return &n+s;` instead it works as expected.  So it seems
as if it is assuming that because new returns a const value it can
elide it, despite the fact that new contains other side effects (in
this case the printf statement).

On Wed, Feb 21, 2018 at 12:29 PM, Sam Clegg <sbc at google.com> wrote:
> 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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