http://gcc.gnu.org/bugzilla/show_bug.cgi?id=55266
Bug #: 55266 Summary: vector expansion: 36 movs for 4 adds Classification: Unclassified Product: gcc Version: 4.8.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: middle-end AssignedTo: unassig...@gcc.gnu.org ReportedBy: gli...@gcc.gnu.org Target: x86_64-linux-gnu I already mentioned this example, but I don't think it is in any PR: typedef double vec __attribute__((vector_size(4*sizeof(double)))); void f(vec*x){ *x+=*x+*x; } compiled with -S -O3 -msse4, produces 4 add insns (normal), and 36 mov insns, which is a bit much... For comparison, this should be equivalent to the following code, which generates only 6 mov insn: typedef double vec __attribute__((vector_size(2*sizeof(double)))); void f(vec*x){ x[0]+=x[0]+x[0]; x[1]+=x[1]+x[1]; } One minor enhancement would be to have fold_ternary handle BIT_FIELD_REF of CONSTRUCTOR of vectors (I think it is already tracked elsewhere, though I couldn't find it). But the main issue is with copying these fake vectors. Their fake "registers" are in memory, and copying between those (4 times 2 movs going through rax in DImode, I assume it is faster than going through xmm registers?) isn't optimized away. In this example, the content of *x is first copied to a fake register. Then V2DF parts are extracted, added, and put in memory. That fake register is now copied to a new fake register. V2DF are taken from it, added to the V2DF that were still there, and stored to memory. And that is finally copied to the memory location x. I don't know how that should be improved. Maybe the vector lowering pass should go even further, turn the first program into the second one, and not leave any extra long vectors for the back-end to handle? It doesn't seem easy to optimize in the back-end, too late. Or maybe something can be done at expansion time?