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Diffstat (limited to 'docs/HistoricalNotes/2000-12-06-EncodingIdea.txt')
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diff --git a/docs/HistoricalNotes/2000-12-06-EncodingIdea.txt b/docs/HistoricalNotes/2000-12-06-EncodingIdea.txt new file mode 100644 index 0000000000..8c452924dd --- /dev/null +++ b/docs/HistoricalNotes/2000-12-06-EncodingIdea.txt @@ -0,0 +1,30 @@ +From: Chris Lattner [mailto:sabre@nondot.org] +Sent: Wednesday, December 06, 2000 6:41 PM +To: Vikram S. Adve +Subject: Additional idea with respect to encoding + +Here's another idea with respect to keeping the common case instruction +size down (less than 32 bits ideally): + +Instead of encoding an instruction to operate on two register numbers, +have it operate on two negative offsets based on the current register +number. Therefore, instead of using: + +r57 = add r55, r56 (r57 is the implicit dest register, of course) + +We could use: + +r57 = add -2, -1 + +My guess is that most SSA references are to recent values (especially if +they correspond to expressions like (x+y*z+p*q/ ...), so the negative +numbers would tend to stay small, even at the end of the procedure (where +the implicit register destination number could be quite large). Of course +the negative sign is reduntant, so you would be storing small integers +almost all of the time, and 5-6 bits worth of register number would be +plenty for most cases... + +What do you think? + +-Chris + |