http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59159
Bug ID: 59159
Summary: Need opaque pass-through as optimization barrier
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Severity: enhancement
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
(category:c because I am asking for a built-in, but this is more about
middle/back-end)
Since gcc doesn't support FENV, I often need to insert optimization barriers in
my code that hide things from gcc optimizers, so it doesn't perform constant
propagation, or replace x*-y with -x*y, or move operations across fesetround,
etc. Note that speed is still very important, doing a store/read on a volatile
variable (the most portable optimization barrier) can make the whole
application 15% slower.
One common way to do that is to use an inline asm like:
asm volatile ("", "+g"(x));
(4.9 is the first version where this doesn't ICE all over the place on x86, use
"+m" earlier)
so no code is emitted but the value of x is hidden from the compiler. However,
the "g" constraint doesn't include FP registers. Depending if the code is
compiled for power, sparc, itanium, arm or x86_64, I need to add either "d",
"e", "f", "w", or "x" (yeah, there aren't 2 using the same letter). It looks
like there isn't even a letter for x87 (PR 59157). Using the more general "+X"
is tempting but ICEs (PR 59155).
There is at least one case this inline asm approach can never handle optimally:
constants. We need the variable as asm output or it won't have any effect. But
the constants are in a read-only location that is not suitable for an output
operand, so with "+m" gcc copies the constant to another memory location.
Ideally, there would be a __builtin_opaque built-in that does the right thing
and I wouldn't have to think about it...
