[llvm-dev] Aliasing rules difference between GCC and Clang (original) (raw)

Friedman, Eli via llvm-dev llvm-dev at lists.llvm.org
Thu Sep 20 12:25:07 PDT 2018

On 9/20/2018 8:55 AM, Jonas Paulsson via llvm-dev wrote:

Hi, I found a difference between Clang and GCC in alias handling. This was with a benchmark where Clang was considerably slower, and in a hot function which does many more loads from the same address due to stores between the uses. In other words, a value is loaded and used, another value is stored, and then the first value is loaded once again before its second use. This happens many times, with three loads instead of one for each value. GCC only emits one load. The values are the arguments to this function: void su3projector( su3vector *a, su3vector *b, su3matrix *c ){ register int i,j; register double tmp,tmp2; for(i=0;i<3;i++)for(j=0;j<3;j++){_ _tmp2 = a->c[i].real * b->c[j].real; tmp = a->c[i].imag * b->c[j].imag; c->e[i][j].real = tmp + tmp2; tmp2 = a->c[i].real * b->c[j].imag; tmp = a->c[i].imag * b->c[j].real; c->e[i][j].imag = tmp - tmp2; } } The types are: typedef struct { complex e[3][3]; } su3matrix; typedef struct { complex c[3]; } su3vector; So the question here is if the su3vector and su3matrix pointers may alias? If they may alias, then clang is right in reloading after each store. If the standard says they cannot alias, then gcc is right in only loading the values once each. It seems to me that either GCC is too aggressive or LLVM is too conservative, but I don't know which one it is... As far as I understand, there is the fact of the different struct types of the arguments (which means they cannot alias), but also the question if su3vector is included in su3matrix (which would mean they may alias).

clang currently emits relatively conservative TBAA info... see CodeGenFunction::EmitLValueForField for the struct handling.  It should be straightforward to add equivalent handling for array indexing.

Not sure about the correctness off the top of my head.

-Eli

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