I'd like to know if gcc has implemented some generic way to help
optimizer job by allowing programmers to specify assumptions (or
constraints).
This is somewhat different from assertions: suppose we have this simple
code:
#include <assert.h>
int p(int a) {
assert(a > 0);
return a <= 0;
}
Compiling with #undef NDEBUG the code generated has a test for a's sign,
a branch for failed assertion and a branch to unconditionally return 0.
Compiling with #define NDEBUG the code generated has a test for a's sign
and the congruent return of 0 and 1.
Neither of this two compilations give the maximum optimization feasible.
If exists a way to write:
int p(int a) {
assume(a > 0);
return a <= 0;
}
this would be compiled with #define NDEBUG as if the assume was an
assert, but with #undef NDEBUG as an unconditional return 0.
In this way we'll get the maximum optimization.
This is a very simple example, but I guess that everyone get the many
possibilities that these gives.
Just to give another examples and to link this to a current thread:
assume(x != INT_MIN || y != -1);
return x % y;
Can be safely compiled with current inexact code also on ppc, i386, x86-64.
Of course the infrastructure is already present in current gcc optimizer
implementation, what I like to know is if there is some way to access it
from compiled code (by example using specific builtin_).
