[llvm-dev] Redundant load in llvm's codegen compares to gcc when accessing escaped pointer?

[Chris Lattner via llvm-dev]

> On Mar 15, 2016, at 7:58 AM, Chuang-Yu Cheng via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
> 
> Please look at this c code:
> 
> typedef struct _PB {
>   void* data;  /* required.*/
>   int   f1_;
>   float f2_;
> } PB;
> 
> PB** bar(PB** t);
> 
> void qux(PB* c) {
>   bar(&c);              /* c is escaped because of bar */
>   c->f1_ = 0;
>   c->f2_ = 0.f;
> }
> 
> // gcc-5.2.1 with -fno-strict-aliasing -O2 on x86
> call    bar
> movq    8(%rsp), %rax
> movl    $0, 8(%rax)
> movl    $0x00000000, 12(%rax)
> 
> // llvm 3.9.0 with -fno-strict-aliasing -O2 on x86
> callq    bar
> movq    (%rsp), %rax
> movl    $0, 8(%rax)
> movq    (%rsp), %rax
> movl    $0, 12(%rax)
> 
> You can see that llvm load "c" twice, but gcc only load "c" once.
> Of course, in bar function, you may do something very dangerous, e.g.
> 
> PB** bar(PB** t) {
>    *t = (PB*) t;
> }
> 
> But gcc doesn't care bar's definition.
> Is llvm too conservative, or gcc too aggressive in this pattern?

In my opinion, in the face of -fno-strict-aliasing, GCC is being too aggressive.  It would be interesting to hear what they think.

-Chris