In Win32, there is an API call called “MulDiv”:<\/p>\n
The MulDiv function multiplies two 32-bit values and then divides the 64-bit result by a third 32-bit value. The return value is rounded up or down to the nearest integer.<\/p>\n
int WINAPI MulDiv(
\nint nNumber,
\nint nNumerator,
\nint nDenominator
\n);<\/p>\n
If a divide overflow (including divide by zero) occurs, MulDiv returns -1. (Stupidly, there’s no way to actually check whether the result truly is -1 instead of an error.)<\/p>\n
How do we implement this in x86-32? The official version does not use structured exception handling to simply catch the exception and handle it. MulDiv actually checks for overflow beforehand and never causes an exception.<\/p>\n
The official implementation is several pages long, and I think we could do much better.<\/p>\n
If this were the unsigned case, the entire function would be simple:<\/p>\n
mov eax, [esp+4]
\nmul dword ptr [esp+8]
\ncmp edx, [esp+12]
\njae overflow
\ndiv dword ptr [esp+12]
\nret 12
\noverflow:
\nor eax, -1
\nret 12<\/p>\n","protected":false},"excerpt":{"rendered":"
In Win32, there is an API call called “MulDiv”: The MulDiv function multiplies two 32-bit values and then divides the 64-bit result by a third 32-bit value. The return value is rounded up or down to the nearest integer. int WINAPI MulDiv( int nNumber, int nNumerator, int nDenominator ); If a divide overflow (including divide … 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-17","post","type-post","status-publish","format-standard","hentry","category-puzzle"],"_links":{"self":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/17","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=17"}],"version-history":[{"count":0,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=\/wp\/v2\/posts\/17\/revisions"}],"wp:attachment":[{"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=17"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=17"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.pagetable.com\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=17"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}