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