Aliasing rules difference between GCC and Clang

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 su3_projector( su3_vector *a, su3_vector *b, su3_matrix *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]; } su3_matrix;
typedef struct { complex c[3]; } su3_vector;

So the question here is if the su3_vector and su3_matrix 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 su3_vector is included in su3_matrix (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];
} STRUCT_TY;

double e = 0.0;
STRUCT_TY *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 EL``EMENT``` 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:
.cfi_startproc
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

``

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

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

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.

This was between a/b and c, not between a and b. Could you explain your opinion a bit more in detail, please?

/Jonas

Oh indeed, sorry, you are doing an outer product on a and b, storing the result in c, and these cannot alias (or they should not, if you do make they alias, it’s your responsibility and I think you can get UB).
So clang should do better indeed.

Regards,

Hi Matthieu,

Oh indeed, sorry, you are doing an outer product on a and b, storing the result in c, and these cannot alias (or they should not, if you do make they alias, it’s your responsibility and I think you can get UB).
So clang should do better indeed.

It would be very nice if you could motivate this in detail with references to the standard. Is it enough that the two struct types have different tags? It might possibly be argued that the matrix contains the vector struct type, and therefore they may alias.
Why do you think this is not the case in this example?

/Jonas

There are many discussions about the strict aliasing undefined behavior:
https://en.wikipedia.org/wiki/Pointer_aliasing

https://gist.github.com/shafik/848ae25ee209f698763cffee272a58f8

https://stackoverflow.com/questions/98650/what-is-the-strict-aliasing-rule

There is also mention of this on cppreference: https://en.cppreference.com/w/cpp/language/reinterpret_cast

Cheers,

Matthieu