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

Philip Reames via llvm-commits llvm-commits at lists.llvm.org
Wed Feb 21 15:02:43 PST 2018



On 02/21/2018 02:28 PM, Sam Clegg wrote:
> On Wed, Feb 21, 2018 at 2:20 PM, Philip Reames
> <listmail at philipreames.com> wrote:
>>
>> On 02/21/2018 12:44 PM, Sam Clegg via llvm-commits wrote:
>>> 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).
>> Probably not the actual reasoning, just FYI.  If we can show we returning a
>> value derived from an alloca, that's UB.  I'd phrase this as simply a missed
>> optimization.  (i.e. an offset from the alloca is still, by assumption,
>> within the alloca and thus UB)
> This is not an alloca but a static.  it behaves the same if I move
> "int n" out into file scope.
Ah, you're right.  I misread.  Sorry!
>
>
>
>>> 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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