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Date: Wed, 14 Mar 2018 20:10:09 -0700 (PDT)
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Subject: Re: Ada case-statement
From: "Dan'l Miller" <optikos@verizon.net>
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Xref: reader02.eternal-september.org comp.lang.ada:50983
Date: 2018-03-14T20:10:09-07:00
List-Id: <comp.lang.ada>

On Wednesday, March 14, 2018 at 7:57:57 PM UTC-5, Robert Eachus wrote:
> The compiler probably will convert to an if statement,=20

For dense case statements that utilize all or most of an enumerated type's =
enumerated values, a jump table is the most likely on most processors to ac=
hieve O(1) growth [instead of if-statement-based O(n) growth, where n is th=
e quantity of enumerated values in an enumerated type].

https://stackoverflow.com/questions/39875452/assembly-jump-branch-lookup-ta=
bles-instead-of-lots-of-cmp-je

In IA-32, a jump table can be transliterated fairly directly from a case-st=
atement over a dense utilization of an enumerated type.  Indeed, if the qua=
ntity of enumerated values in the enumerated type is small enough, a case s=
tatement that sparsely utilizes only a relative few of the enumerated value=
s also can be transliterated to a jump table, where the unmentioned enumera=
ted values conflate to unhandled case.
jmp  [jump_table + si]
jump_table:
   dw   .F_1,  .F_2, ...