| Message ID | 876014ld0p.fsf@arm.com |
|---|---|
| State | New |
| Headers | show |
| Series | Fix ceil_log2(0) (PR 86644) | expand |
On 07/24/2018 12:11 PM, Richard Sandiford wrote: > This PR shows a pathological case in which we try SLP vectorisation on > dead code. We record that 0 bits of the result are enough to satisfy > all users (which is true), and that led to precision being 0 in: > > static unsigned int > vect_element_precision (unsigned int precision) > { > precision = 1 << ceil_log2 (precision); > return MAX (precision, BITS_PER_UNIT); > } > > ceil_log2 (0) returned 64 rather than 0, leading to 1 << 64, which is UB. > > Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu. > OK to install? > > Richard > > > 2018-07-24 Richard Sandiford <richard.sandiford@arm.com> > > gcc/ > * hwint.c (ceil_log2): Fix comment. Return 0 for 0. OK Jeff
Index: gcc/hwint.c =================================================================== --- gcc/hwint.c 2018-05-02 08:38:14.433364094 +0100 +++ gcc/hwint.c 2018-07-24 19:09:03.522774662 +0100 @@ -60,12 +60,12 @@ floor_log2 (unsigned HOST_WIDE_INT x) return t; } -/* Given X, an unsigned number, return the largest Y such that 2**Y >= X. */ +/* Given X, an unsigned number, return the least Y such that 2**Y >= X. */ int ceil_log2 (unsigned HOST_WIDE_INT x) { - return floor_log2 (x - 1) + 1; + return x == 0 ? 0 : floor_log2 (x - 1) + 1; } /* Return the logarithm of X, base 2, considering X unsigned,
This PR shows a pathological case in which we try SLP vectorisation on dead code. We record that 0 bits of the result are enough to satisfy all users (which is true), and that led to precision being 0 in: static unsigned int vect_element_precision (unsigned int precision) { precision = 1 << ceil_log2 (precision); return MAX (precision, BITS_PER_UNIT); } ceil_log2 (0) returned 64 rather than 0, leading to 1 << 64, which is UB. Tested on aarch64-linux-gnu, aarch64_be-elf and x86_64-linux-gnu. OK to install? Richard 2018-07-24 Richard Sandiford <richard.sandiford@arm.com> gcc/ * hwint.c (ceil_log2): Fix comment. Return 0 for 0.