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util: Re-indent util_sign_extend, comment, and add asserts 

The current implementation depends on both of these things for
correctness.  If width > 64, you get UB wrap-around and, if
val >= (1 << width), the subtract trick won't work.

Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/17214>
This commit is contained in:
Jason Ekstrand 2022-06-23 12:23:24 -05:00 committed by Marge Bot
parent 20016aa8f6
commit 6fee4584ea
1 changed files with 11 additions and 7 deletions
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18
src/util/u_math.h
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@ -580,17 +580,21 @@ util_bswap16(uint16_t n)
}
/**
* Extend sign.
* Sign-extend a number
*
* The bit at position `width - 1` is replicated to all the higher bits.
* This assumes and asserts that the value fits into `width` bits.
*/
static inline int64_t
util_sign_extend(uint64_t val, unsigned width)
{
assert(width > 0);
if (val & (UINT64_C(1) << (width - 1))) {
return -(int64_t)((UINT64_C(1) << width) - val);
} else {
return val;
}
assert(width > 0 && width <= 64);
assert(width == 64 || val < (1ull << width));
if (val & (UINT64_C(1) << (width - 1))) {
return -(int64_t)((UINT64_C(1) << width) - val);
} else {
return val;
}
}
static inline void*