Messages in this thread | | | From | Rasmus Villemoes <> | Date | Tue, 27 Jun 2017 09:58:52 +0200 | Subject | Re: [PATCH 2/3] Turn bitmap_set and bitmap_clear into memset when possible |
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> It's actually slightly less efficient in the caller (although obviously > memset() is going to execute faster than bitmap_set()). Partly because > x86 made some odd choices about the behaviour of an 8-bit move instruction > (it leaves the upper 24 bits intact, rather than zeroing them, so gcc > has to use a 32-bit move instruction to put 0xff into the second argument > to memset()),
Heh, I thought gcc knew and made full use of the semantics of memset, so that only the low byte matters. I suppose there might be architectures where passing -1 is slightly cheaper (at least in code size) than 255... [quick checking] indeed, on x86_64, there's no change in the generated code, but on 32 bit, gcc ends up doing
6a ff push $0xffffffff
instead of
68 ff 00 00 00 push $0xff
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