Hello,
I've come across an issue that I would have thought there would be a
builtin for, but perhaps that's just wishful thinking. I'd like to be
able to write something like this:
if (__builtin_expr_is_true(x > 0))
... // one thing
else
... // something else
This stems from trying to clean up the mess of bit operation helpers in Xen.
On x86, __builtin_ffs() doesn't have great code generation. This is a
consequence of the BSF instruction having miserable semantics, and the
builtin emits code with a branch or cmov to compensate for undefined
case of passing 0 in.
On x86_64 at least, Intel and AMD have made enough guarantees in writing
to allow a condition-less form:
mov $-1, %dst
bsf %src, %dst
add $1, %dst
which is good, but not great. It is common to have an __ffs() variant
which states that a src of 0 is undefined, and while this makes a
reasonable improvement to the code generation within loops, it's still
not great to rely on the programmer to get this right.
A common pattern to find is something like:
while (x) {
int b = ffs(x);
... // do something with x and b
where range analysis can know that x is nonzero. Indeed, the builtin
manages to spot this, and emits a condition-less form too.
However, doing this for a local implementation of ffs() doesn't work. With:
unsigned int my_ffs(unsigned int x)
{
int res;
if (x) {
asm ("bsf ..." : "=r" (res) : "rm" (x));
} else {
res = -1;
asm ("bsf ..." : "+r" (res) : "rm" (x));
}
return res + 1;
}
the while() example above really does get generated with ideal form.
However, in general code where the value of x is unknown, the entire
if/else chain is emitted, which is strictly worse than just emitting the
else case which is the safe catch-all.
I suppose that what I'm looking for is something a little like
__builtin_constant_p() which can either be used in a straight if(), or
in a __builtin_choose_expr().
Anyway - is there a way of doing this that I've managed to overlook?
~Andrew
