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[patch] avoid tagging integers before right shifting it #5824

vicuna opened this issue Nov 14, 2012 · 1 comment

[patch] avoid tagging integers before right shifting it #5824

vicuna opened this issue Nov 14, 2012 · 1 comment


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@vicuna vicuna commented Nov 14, 2012

Original bug ID: 5824
Reporter: @chambart
Status: closed (set by @xavierleroy on 2015-12-11T18:08:24Z)
Resolution: fixed
Priority: normal
Severity: feature
Fixed in version: 4.01.0+dev
Category: back end (clambda to assembly)

Bug description

When right shifting a value, the rightmost bit can be ignored:

let f a =
(a lsl 3) lsr 2

is compiled as:

(or (>>u (+ (<< (+ a/1009 -1) 3) 1) 2) 1))

but could be:

(or (>>u (<< (+ a/1009 -1) 3) 2) 1))

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@vicuna vicuna commented Nov 17, 2012

Comment author: @xavierleroy

Well spotted. Patch applied in trunk, revision 13096. Note that the checks "n > 0" in lsr_int and asr_int are probably unnecessary, since the code generated by Plsrint and Pasrint always does an "or 1" afterwards, restoring the correct tag bit even if n = 0. But n = 0 never happens in well-written code, so we can be conservative.

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