{"action":"created","issue":{"url":"https://api.github.com/repos/rust-lang/rust/issues/49842","repository_url":"https://api.github.com/repos/rust-lang/rust","labels_url":"https://api.github.com/repos/rust-lang/rust/issues/49842/labels{/name}","comments_url":"https://api.github.com/repos/rust-lang/rust/issues/49842/comments","events_url":"https://api.github.com/repos/rust-lang/rust/issues/49842/events","html_url":"https://github.com/rust-lang/rust/issues/49842","id":312913088,"number":49842,"title":"`mul_add` documentation is inaccurate","user":{"login":"vks","id":33460,"avatar_url":"https://avatars2.githubusercontent.com/u/33460?v=4","gravatar_id":"","url":"https://api.github.com/users/vks","html_url":"https://github.com/vks","followers_url":"https://api.github.com/users/vks/followers","following_url":"https://api.github.com/users/vks/following{/other_user}","gists_url":"https://api.github.com/users/vks/gists{/gist_id}","starred_url":"https://api.github.com/users/vks/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/vks/subscriptions","organizations_url":"https://api.github.com/users/vks/orgs","repos_url":"https://api.github.com/users/vks/repos","events_url":"https://api.github.com/users/vks/events{/privacy}","received_events_url":"https://api.github.com/users/vks/received_events","type":"User","site_admin":false},"labels":[],"state":"open","locked":false,"assignee":null,"assignees":[],"milestone":null,"comments":1,"created_at":"2018-04-10T13:03:38Z","updated_at":"2018-04-10T17:17:49Z","closed_at":null,"author_association":"CONTRIBUTOR","body":"According to the [documentation of `mul_add`](https://doc.rust-lang.org/std/primitive.f64.html#method.mul_add):\r\n> This produces a more accurate result with better performance than a separate multiplication operation followed by an add.\r\n\r\nThis seems not to be true in general. On my machine, `a.mul_add(b, c)` is slower than `a * b + c` when compiling without `target-cpu=native`."},"comment":{"url":"https://api.github.com/repos/rust-lang/rust/issues/comments/380179596","html_url":"https://github.com/rust-lang/rust/issues/49842#issuecomment-380179596","issue_url":"https://api.github.com/repos/rust-lang/rust/issues/49842","id":380179596,"user":{"login":"rkruppe","id":2311707,"avatar_url":"https://avatars2.githubusercontent.com/u/2311707?v=4","gravatar_id":"","url":"https://api.github.com/users/rkruppe","html_url":"https://github.com/rkruppe","followers_url":"https://api.github.com/users/rkruppe/followers","following_url":"https://api.github.com/users/rkruppe/following{/other_user}","gists_url":"https://api.github.com/users/rkruppe/gists{/gist_id}","starred_url":"https://api.github.com/users/rkruppe/starred{/owner}{/repo}","subscriptions_url":"https://api.github.com/users/rkruppe/subscriptions","organizations_url":"https://api.github.com/users/rkruppe/orgs","repos_url":"https://api.github.com/users/rkruppe/repos","events_url":"https://api.github.com/users/rkruppe/events{/privacy}","received_events_url":"https://api.github.com/users/rkruppe/received_events","type":"User","site_admin":false},"created_at":"2018-04-10T17:17:49Z","updated_at":"2018-04-10T17:17:49Z","author_association":"CONTRIBUTOR","body":"cc #44805, dedicated FMA instructions are not universally available and we can't very well replace `a.mul_add(b, c)` with `a * b + c` when no such instruction exists (it rounds differently and also has different behavior when NaNs or infinities are involved)."}}
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