\nenum Axis\n{\n XAxis,\n YAxis,\n ZAxis,\n};\n\/\/ code earlier in the function ensure that\n\/\/ \"axis\" is always a valid axis\nint newPosition;\nswitch (axis)\n{\ncase XAxis:\n newPosition = m_position[XAxis] + amount;\n if (newPosition < m_minPosition[XAxis])\n newPosition = m_minPosition[XAxis];\n if (newPosition > m_maxPosition[XAxis])\n newPosition = m_maxPosition[XAxis];\n m_position[XAxis] = amount;\n break;\ncase YAxis:\n newPosition = m_position[YAxis] + amount;\n if (newPosition < m_minPosition[YAxis])\n newPosition = m_minPosition[YAxis];\n if (newPosition > m_maxPosition[YAxis])\n newPosition = m_maxPosition[YAxis];\n m_position[YAxis] = amount;\n break;\ncase ZAxis:\n newPosition = m_position[ZAxis] + amount;\n if (newPosition < m_minPosition[ZAxis])\n newPosition = m_minPosition[ZAxis];\n if (newPosition > m_maxPosition[XAxis])\n newPosition = m_maxPosition[XAxis];\n m_position[ZAxis] = amount;\n break;\n}\n<\/pre>\nAs we all know,\nspecial-case code is faster than general-purpose code.\nInstead of writing slow general-purpose code:\n<\/p>\n
\n newPosition = m_position[axis] + amount;\n if (newPosition < m_minPosition[axis])\n newPosition = m_minPosition[axis];\n if (newPosition > m_maxPosition[axis])\n newPosition = m_maxPosition[axis];\n m_position[axis] = amount;\n<\/pre>\n\nwe unroll it into a switch statement, thereby generating\nhighly-optimized\nspecial-purpose code, one for each axis.\n<\/p>\n
\nWhat makes this anti-pattern particularly frustrating is that you\ncannot tell at a glance whether all the cases really are the same\n(just with different axes).\n<\/p>\n
\nIn fact, they aren’t.\n<\/p>\n
\nIf you look closely, you’ll see that we check the\nnew Z-position against the X-axis maximum rather than the Z-axis\nmaximum.\nIf you’re reading this code, you now start to wonder,\n“Is this a copy\/paste bug, or is there some reason that\nwe really do want to check the Z-position against the X-axis minimum?”\n<\/p>\n
\nA variation on the unrolled-switch<\/i> is the\nunrolled-if<\/i>, used if the item you want to unroll\ncannot be used in a switch<\/i> statement:\n<\/p>\n\nFruitBasket *BananaBasket;\nFruitBasket *AppleBasket;\nFruitBasket *PearBasket;\nFruitBasket *MangoBasket;\nif (basket == BananaBasket) {\n if (!BananaBasket->IsEmpty()) {\n fruit = BananaBasket->TakeFruit();\n if (HaveKnife()) {\n TakeKnife();\n fruit->Peel();\n fruit->Slice();\n fruit->Eat();\n ReplaceKnife();\n } else {\n BananaBasket->AddFruit(fruit);\n }\n }\n} else if (basket == AppleBasket) {\n if (!AppleBasket->IsEmpty()) {\n fruit = AppleBasket->TakeFruit();\n if (HaveKnife()) {\n TakeKnife();\n fruit->Peel();\n fruit->Slice();\n fruit->Eat();\n ReplaceKnife();\n } else {\n AppleBasket->AddFruit(fruit);\n }\n }\n} else if (basket == PearBasket) {\n if (!PearBasket->IsEmpty()) {\n fruit = PearBasket->TakeFruit();\n if (HaveKnife()) {\n TakeKnife();\n fruit->Slice();\n fruit->Eat();\n ReplaceKnife();\n } else {\n PearBasket->AddFruit(fruit);\n }\n }\n} else if (basket == MangoBasket) {\n if (!MangoBasket->IsEmpty()) {\n fruit = MangoBasket->TakeFruit();\n if (HaveKnife()) {\n TakeKnife();\n fruit->Peel();\n fruit->Slice();\n fruit->Eat();\n ReplaceKnife();\n } else {\n BananaBasket->AddFruit(fruit);\n }\n }\n}\n<\/pre>\n\nWhen I pointed out in an aside to the customer that this could\nbe simplified (after fixing the copy\/paste errors) to\n<\/p>\n
\nif (!basket->IsEmpty()) {\n fruit = basket->TakeFruit();\n if (HaveKnife()) {\n TakeKnife();\n fruit->Peel();\n fruit->Slice();\n fruit->Eat();\n ReplaceKnife();\n } else {\n basket->AddFruit(fruit);\n }\n}\n<\/pre>\n\nthe response was, “Hey, that’s a neat trick.\nI didn’t realize you could do that.”\n<\/p>\n
\nI wonder if this person also programs loops like this:\n<\/p>\n
\nswitch (limit)\n{\ncase 0:\n break;\ncase 1:\n do_something(array[0]);\n break;\ncase 2:\n for (int i = 0; i < 2; i++) do_something(array[i]);\n break;\ncase 3:\n for (int i = 0; i < 3; i++) do_something(array[i]);\n break;\ncase 4:\n for (int i = 0; i < 4; i++) do_something(array[i]);\n break;\ncase 5:\n for (int i = 0; i < 5; i++) do_something(array[i]);\n break;\ncase 6:\n for (int i = 0; i < 6; i++) do_something(array[i]);\n break;\n...\ncase 999:\n for (int i = 0; i < 999; i++) do_something(array[i]);\n break;\ndefault:\n FatalError(\"Need more cases to handle larger array\");\n break;\n}\n<\/pre>\n","protected":false},"excerpt":{"rendered":"Over the years, I’ve seen a bunch of coding anti-patterns. I figured maybe I’ll share a few. Today, I’ll introduce what I’m calling the unrolled-switch anti-pattern, also known as “Specialization is always faster, right?” enum Axis { XAxis, YAxis, ZAxis, }; \/\/ code earlier in the function ensure that \/\/ “axis” is always a valid […]<\/p>\n","protected":false},"author":1069,"featured_media":111744,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[1],"tags":[25],"class_list":["post-7943","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-oldnewthing","tag-code"],"acf":[],"blog_post_summary":"
Over the years, I’ve seen a bunch of coding anti-patterns. I figured maybe I’ll share a few. Today, I’ll introduce what I’m calling the unrolled-switch anti-pattern, also known as “Specialization is always faster, right?” enum Axis { XAxis, YAxis, ZAxis, }; \/\/ code earlier in the function ensure that \/\/ “axis” is always a valid […]<\/p>\n","_links":{"self":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/7943","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/users\/1069"}],"replies":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/comments?post=7943"}],"version-history":[{"count":0,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/posts\/7943\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media\/111744"}],"wp:attachment":[{"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/media?parent=7943"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/categories?post=7943"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/devblogs.microsoft.com\/oldnewthing\/wp-json\/wp\/v2\/tags?post=7943"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}