In C, the ! (“logical NOT”) operator used on a value x evaluates to 0 when x is not 0, and 1 when x is 0. In other words, it’s equivalent to the following C:<\/p>\n
(x == 0) ? 1 : 0<\/p>\n
How should this be implemented in x86 assembly language, when “x” is already in a register? The target register can either be the same one, or it can be a different one. I didn’t try too hard and got 7 bytes; it can probably be made better. On other CPUs, it can be done in a single instruction. For example, in MIPS: “sltiu dest, src, 1”.<\/p>\n
Note that this is about the case where the compiler doesn’t know how the result of the ! is used, as in “return !x;” in a non-inlined function. Cases like “if (!x)” are simpler.<\/p>\n
(If you want to share how easily it can be done on *your* favorite CPU, please post a comment as well!)<\/p>\n","protected":false},"excerpt":{"rendered":"
In C, the ! (“logical NOT”) operator used on a value x evaluates to 0 when x is not 0, and 1 when x is 0. In other words, it’s equivalent to the following C: (x == 0) ? 1 : 0 How should this be implemented in x86 assembly language, when “x” is already … 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":[26],"tags":[],"class_list":["post-13","post","type-post","status-publish","format-standard","hentry","category-puzzle"],"_links":{"self":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/13","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=13"}],"version-history":[{"count":0,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/13\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=13"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=13"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=13"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}