I thought of an optimization that compilers for most CPUs could do that I think should be implemented. Let’s say you have C code like this:<\/p>\n
if ((variable == 4) || (variable == 6)) { stuff }<\/p>\n
Any existing compiler I know of would compile into something like this (eax is variable):<\/p>\n
cmp eax, 4
\njz label
\ncmp eax, 6
\njz label<\/p>\n
That’s a bad way to do it. It should be implemented more like this:<\/p>\n
or eax, 2
\ncmp eax, 6
\njz label<\/p>\n
This saves one instruction, but more importantly, it saves a conditional branch. Conditional branches can be expensive on modern hardware. This will work on any similar “if” statement whose constants differ by a single bit.<\/p>\n
Even if you must copy eax to another register first, it removes a conditional branch, which is probably a win. The situations in which to do this are more complicated and must take into account many things.<\/p>\n
For constants that are 1 apart but not 1 bit apart, like 7 and 8, you can do:<\/p>\n
sub eax, 7 \/\/ use dec eax in 1\/2 case
\ntest eax, 0xFFFFFFFE \/\/ this is a 3 byte opcode
\njz label<\/p>\n
This also works for 0 and -1: increment eax first.<\/p>\n
myria<\/p>\n","protected":false},"excerpt":{"rendered":"
I thought of an optimization that compilers for most CPUs could do that I think should be implemented. Let’s say you have C code like this: if ((variable == 4) || (variable == 6)) { stuff } Any existing compiler I know of would compile into something like this (eax is variable): cmp eax, 4 … Read more<\/a><\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[32,38],"tags":[],"class_list":["post-10","post","type-post","status-publish","format-standard","hentry","category-tricks","category-x86"],"_links":{"self":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/10","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=10"}],"version-history":[{"count":0,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/10\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=10"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=10"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=10"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}