{"id":21323,"date":"2022-11-13T01:59:20","date_gmt":"2022-11-12T20:29:20","guid":{"rendered":"https:\/\/jassweb.com\/solved\/solved-does-x-x-where-x-is-an-unsigned-integer-invoke-ub\/"},"modified":"2022-11-13T01:59:20","modified_gmt":"2022-11-12T20:29:20","slug":"solved-does-x-x-where-x-is-an-unsigned-integer-invoke-ub","status":"publish","type":"post","link":"https:\/\/jassweb.com\/solved\/solved-does-x-x-where-x-is-an-unsigned-integer-invoke-ub\/","title":{"rendered":"[Solved] Does x ^= x & -x; where x is an unsigned integer invoke UB?"},"content":{"rendered":"

[ad_1]
\n<\/p>\n

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<\/div>\n
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The behavior of this expression is well defined.<\/p>\n

Constructs similar to x = x + 1<\/code> are allowed because x<\/code> isn’t assigned a value until all other subexpressions are evaulated. The same applies in this case.<\/p>\n

There is also no problem with -x<\/code> because the expression has unsigned type and thus has well defined wraparound behavior as opposed to overflowing.<\/p>\n

Section 6.5.3.3p3 of the C standard<\/a> regarding the unary -<\/code> operator states:<\/p>\n

\n

The result of the unary -<\/code> operator is the negative of its (promoted) operand. The integer promotions are performed on the operand, and the result has the promoted type.<\/p>\n<\/blockquote>\n

So since no promotion occurs the type remains unsigned<\/code> throughout the expression. Though not explicitly stated in the standard, -x<\/code> is effectively the same as 0 - x<\/code>.<\/p>\n

For the specific case of INT_MIN<\/code> being passed to this function, it has type int<\/code> and is outside of the range of unsigned<\/code>, so it is converted when passed to the function. This results in the signed value -2,147,483,648 being converted to the unsigned value 2,147,483,648 (which in two’s complement happen to have the same representation, i.e. 0x80000000). Then when -x<\/code> is evaluated, it wraps around resulting in 2,147,483,648.<\/p>\n<\/p><\/div>\n

<\/div>\n<\/div>\n

2<\/span> <\/p><\/div>\n<\/div>\n

[ad_2]<\/p>\n

solved Does x ^= x & -x; where x is an unsigned integer invoke UB? <\/p>\n","protected":false},"excerpt":{"rendered":"

[ad_1] The behavior of this expression is well defined. Constructs similar to x = x + 1 are allowed because x isn’t assigned a value until all other subexpressions are evaulated. The same applies in this case. There is also no problem with -x because the expression has unsigned type and thus has well defined … Read more<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[320],"tags":[324,2796],"class_list":["post-21323","post","type-post","status-publish","format-standard","hentry","category-solved","tag-c","tag-language-lawyer"],"yoast_head":"\n[Solved] Does x ^= x & -x; where x is an unsigned integer invoke UB? - JassWeb<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/jassweb.com\/solved\/solved-does-x-x-where-x-is-an-unsigned-integer-invoke-ub\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"[Solved] Does x ^= x & -x; where x is an unsigned integer invoke UB? - JassWeb\" \/>\n<meta property=\"og:description\" content=\"[ad_1] The behavior of this expression is well defined. 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