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

[Benjamin Kramer via llvm-commits]

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.


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
> > +}
> >
> >
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
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