I’ve seen several instances of code recently that look something like this:<\/p>\n
void madd(int64_t *sum, int32_t x, int32_t y)\r\n{\r\n *sum += x * y;\r\n}<\/pre>\nOr this:<\/p>\n
void func(int64_t);\r\n...\r\n int32_t x, y = ...;\r\n ...\r\n func(x * y);<\/pre>\nUnfortunately, neither of these cases do what the programmer intended.\u00a0 The intended result was to compute the full 64-bit product from multiplying two 32-bit numbers.\u00a0 What gets computed instead is the lower 32-bits of the desired product, sign-extended to 64-bits, which is quite different! The assembly produced by x86-64 GCC at -O2<\/tt> for the first example looks like:<\/p>\n imull %edx, %esi # int32_t multmp = x * y\r\n movslq %esi, %rsi # int64_t exttmp = static_cast<int64_t>(multmp)\r\n addq %rsi, (%rdi) # *sum += exttmp\r\n ret<\/pre>\nIf the full 64-bit product is desired, one of the arguments needs to be cast to a 64-bit value first.<\/p>\n
void madd(int64_t *sum, int32_t x, int32_t y)\r\n{\r\n *sum += static_cast<int64_t>(x) * y;\r\n}<\/pre>\n(The standard-ese for this is that operands are automatically promoted based on the types of the operands, not on the type of the result.\u00a0 Integers smaller than int<\/tt> are promoted to int<\/tt>, which is what you want most of the time. Of course, here we’re dealing with things that are already int<\/tt>-sized[*], so we have to explicitly ask for promotion.)<\/p>\n
which produces the desired:<\/p>\n
movslq %esi, %rsi # int64_t xtmp = static_cast<int64_t>(x)\r\n movslq %edx, %rdx # int64_t ytmp = static_cast<int64_t>(y)\r\n imulq %rdx, %rsi # int64_t multmp = xtmp * ytmp\r\n addq %rsi, (%rdi) # *sum += multmp\r\n ret<\/pre>\nThe above examples are semi-obvious instances, but when dealing with types whose sizes are not specified, similar problems occur. Consider replacing int64_t<\/tt> with off_t<\/tt> and int32_t<\/tt> with size_t<\/tt> in the example above. While such code will mostly work (most files are well under 2GB or 4GB in size), off_t<\/tt> and size_t<\/tt> do not need to be the same size: try compiling sizeof off_t<\/tt> with -D_FILE_OFFSET_BITS=64<\/tt> on your favorite 32-bit Linux sometime.<\/p>\n
[*] Assuming we’re on a fairly standard 32-bit or 64-bit machine, of course.<\/p>\n","protected":false},"excerpt":{"rendered":"
I’ve seen several instances of code recently that look something like this: void madd(int64_t *sum, int32_t x, int32_t y) { *sum += x * y; } Or this: void func(int64_t); … int32_t x, y = …; … func(x * y); Unfortunately, neither of these cases do what the programmer intended.\u00a0 The intended result was to […]<\/p>\n","protected":false},"author":320,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[1],"tags":[],"_links":{"self":[{"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/posts\/117"}],"collection":[{"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/users\/320"}],"replies":[{"embeddable":true,"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/comments?post=117"}],"version-history":[{"count":0,"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/posts\/117\/revisions"}],"wp:attachment":[{"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/media?parent=117"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/categories?post=117"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/blog.mozilla.org\/nfroyd\/wp-json\/wp\/v2\/tags?post=117"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}