• Home
  • Line#
  • Scopes#
  • Navigate#
  • Raw
  • Download
1From: Chris Lattner [mailto:sabre@nondot.org]
2Sent: Wednesday, December 06, 2000 6:41 PM
3To: Vikram S. Adve
4Subject: Additional idea with respect to encoding
5
6Here's another idea with respect to keeping the common case instruction
7size down (less than 32 bits ideally):
8
9Instead of encoding an instruction to operate on two register numbers,
10have it operate on two negative offsets based on the current register
11number.  Therefore, instead of using:
12
13r57 = add r55, r56  (r57 is the implicit dest register, of course)
14
15We could use:
16
17r57 = add -2, -1
18
19My guess is that most SSA references are to recent values (especially if
20they correspond to expressions like (x+y*z+p*q/ ...), so the negative
21numbers would tend to stay small, even at the end of the procedure (where
22the implicit register destination number could be quite large).  Of course
23the negative sign is reduntant, so you would be storing small integers
24almost all of the time, and 5-6 bits worth of register number would be
25plenty for most cases...
26
27What do you think?
28
29-Chris
30
31