https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88770
Peter Cordes <peter at cordes dot ca> changed:
What |Removed |Added
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CC| |peter at cordes dot ca
--- Comment #2 from Peter Cordes <peter at cordes dot ca> ---
Note that mov r64, imm64 is a 10-byte instruction, and can be slow to read from
the uop-cache on Sandybridge-family.
The crap involving OR is clearly sub-optimal, but *if* you already have two
spare call-preserved registers across this call, the following is actually
smaller code-size:
movabs rdi, 21474836483
mov rbp, rdi
movabs rsi, 39743127552
mov rbx, rsi
call test
mov rdi, rbp
mov rsi, rbx
call test
This is more total uops for the back-end though (movabs is still single-uop,
but takes 2 entries the uop cache on Sandybridge-family;
https://agner.org/optimize/). So saving x86 machine-code size this way does
limit the ability of out-of-order exec to see farther, if the front-end isn't
the bottleneck. And it's highly unlikely to be worth saving/restoring two regs
to enable this. (Or to push rdi / push rsi before call, then pop after!)
Setting up the wrong value and then fixing it twice with OR is obviously
terrible and never has any advantage, but the general idea to CSE large
constants isn't totally crazy. (But it's profitable only in such limited cases
that it might not be worth looking for, especially if it's only helpful at -Os)
