https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120032
Feng Xue <fxue at os dot amperecomputing.com> changed:
What |Removed |Added
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CC| |fxue at os dot
amperecomputing.com
--- Comment #8 from Feng Xue <fxue at os dot amperecomputing.com> ---
1. Absorbing 31 substract constant into the array, pattern match fails.
static const unsigned int DeBruijnClz[32] = {31, 22, 30, 21, 18, 10, 29, 2,
20, 17, 15, 13, 9, 6, 28, 1,
23, 19, 11, 3, 16, 14, 7, 24,
12, 4, 8, 25, 5, 26, 27, 0};
val |= val >> 1;
val |= val >> 2;
val |= val >> 4;
val |= val >> 8;
val |= val >> 16;
return DeBruijnClz[(val * 0x07C4ACDDU) >> 27];
2. Use a value who has same LSB as "val" (other bits are zero) via expression
(val - (val >> 1)), pattern match fails.
static const unsigned int DeBruijnClz[32] = {0, 1, 10, 2, 11, 14, 22, 3,
30, 12, 15, 17, 19, 23, 26, 4,
31, 9, 13, 21, 29, 16, 18, 25,
8, 20, 28, 24, 7, 27, 6, 5};
val |= val >> 1;
val |= val >> 2;
val |= val >> 4;
val |= val >> 8;
val |= val >> 16;
return 31 - DeBruijnClz[((val - (val >> 1)) * 0x07C4ACDDU) >> 27];
