On Mon, 4 Jun 2012, Florian Weimer wrote:
On 06/01/2012 01:34 PM, Jakub Jelinek wrote:
Have you looked at the assembly differences with this in?
It's not great.
Here's an example:
void
write(std::vector<float>& blob, unsigned n, float v1, float v2, float v3,
float v4)
{
blob[n] = v1;
blob[n + 1] = v2;
blob[n + 2] = v3;
blob[n + 3] = v4;
}
Would be great if it ended up testing only n and n+3.
__attribute__((__noreturn__)) is not quite strong enough to allow this
optimization, it would require something like
__attribute__((__crashing__)) to let the compiler know that if the
function is called, you don't care what happens to blob. And possibly the
use of a signed n.
Note that even when the optimization would be legal, gcc seems to have a
few difficulties:
extern "C" void fail() __attribute((noreturn));
void write(signed m, signed n)
{
if((n+3)>m) fail();
if((n+2)>m) fail();
if((n+1)>m) fail();
if(n>m) fail();
}
keeps 3 tests.
--
Marc Glisse
