d27b7e6987
When compiling an optabs.ii at -O2 with a release-checking build,
the hottest function in the profile was irange_union. This patch
tries to optimise it a bit. The specific changes are:
- Use quick_push rather than safe_push, since the final number
of entries is known in advance.
- Avoid assigning wi::to_wide & co. to a temporary wide_int,
such as in:
wide_int val_j = wi::to_wide (res[j]);
wi::to_wide returns a wide_int "view" of the in-place INTEGER_CST
storage. Assigning the result to wide_int forces an unnecessary
copy to temporary storage.
This is one area where "auto" helps a lot. In the end though,
it seemed more readable to inline the wi::to_*s rather than
use auto.
- Use to_widest_int rather than to_wide_int. Both are functionally
correct, but to_widest_int is more efficient, for three reasons:
- to_wide returns a wide-int representation in which the most
significant element might not be canonically sign-extended.
This is because we want to allow the storage of an INTEGER_CST
like 0x1U << 31 to be accessed directly with both a wide_int view
(where only 32 bits matter) and a widest_int view (where many more
bits matter, and where the 32 bits are zero-extended to match the
unsigned type). However, operating on uncanonicalised wide_int
forms is less efficient than operating on canonicalised forms.
- to_widest_int has a constant rather than variable precision and
there are never any redundant upper bits to worry about.
- Using widest_int avoids the need for an overflow check, since
there is enough precision to add 1 to any IL constant without
wrap-around.
This gives a ~2% compile-time speed up with the test above.
I also tried adding a path for two single-pair ranges, but it
wasn't a win.
gcc/
* value-range.cc (irange::irange_union): Use quick_push rather
than safe_push. Use widest_int rather than wide_int. Avoid
assigning wi::to_* results to wide*_int temporaries.
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