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

Matthieu Brucher via llvm-dev llvm-dev at lists.llvm.org
Thu Sep 20 14:40:19 PDT 2018

Hi,

I would say that GCC is wrong and should also have a version where a could be equal to b. There is no restrict keyword, so they could be equal.

Cheers,

Matthieu

Le jeu. 20 sept. 2018 à 16:56, Jonas Paulsson via llvm-dev < llvm-dev at lists.llvm.org> a écrit :

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). I made a reduced test case, where the same difference seems to be present. It has just one struct type which contains a matrix of double:s. A store to an element of the struct via a pointer is surrounded with two loads of a global double variable. Only Clang emits two loads. typedef struct { double c[3][3]; } STRUCTTY; double e = 0.0; STRUCTTY *f; int g = 0; void h() { int i = e; f->c[0][i] = g; g = e; } clang -O3-march=z13 : h: # @h # %bb.0: # %entry larl %r1, e ld %f0, 0(%r1) // LOAD E lrl %r2, g cfdbr %r0, 5, %f0 // CONVERT E lgfr %r0, %r0 // EXTEND E cdfbr %f0, %r2 lgrl %r2, f sllg %r3, %r0, 3 std %f0, 0(%r3,%r2) // STORE F ELEMENT ld %f0, 0(%r1) // 2nd LOAD E <<<<<<< cfdbr %r0, 5, %f0 // CONVERT strl %r0, g // 2nd USE br %r14 gcc -O3-march=z13 : h: .LFB0: .cfistartproc larl %r1,e ld %f0,0(%r1) // LOAD E lrl %r2,g lgrl %r3,f cfdbr %r1,5,%f0 // CONVERT E cdfbr %f0,%r2 lgfr %r2,%r1 // EXTEND E sllg %r2,%r2,3 std %f0,0(%r2,%r3) // STORE F ELEMENT strl %r1,g // 2nd USE br %r14 I hope somebody with enough experience and knowledge can guide the way here as this seems to be quite important. /Jonas

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