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Diffstat (limited to 'gas')
| -rw-r--r-- | gas/NOTES | 31 |
1 files changed, 31 insertions, 0 deletions
@@ -99,6 +99,37 @@ easier to maintain, instead of having code in most of the back ends. PIC support. +Torbjorn Granlund <tege@cygnus.com> writes, regarding alpha .align: + + Please make sure the .align directive works as in digital's assembler. + They fill the space with a sequence of "bis $31,$31,$31;ldq_u $31,0($30)" + since these two instructions can dual-issue. Since .align is ued a lot by + gcc, it is an important optimization. + +Torbjorn Granlund <tege@cygnus.com> writes, regarding i386/i486/pentium: + + In a new publication from Intel, "Optimization for Intel's 32 bit + Processors", they recommended code alignment on a 16 byte boundary if that + requires less than 8 bytes of fill instructions. The Pentium is not + affected by such alignment, the 386 wants alignment on a 4 byte boundary. + It is the 486 that is most helped by large alignment. + + Recommended nop instructions: + 1 byte: 90 xchg %eax,%eax + 2 bytes: 8b c0 movl %eax,%eax + 3 bytes: 8d 76 00 leal 0(%esi),%esi + 4 bytes: 8d 74 26 00 leal 0(%esi),%esi + 5 bytes: 8b c0 8d 76 00 movl %eax,%eax; leal 0(%esi),%esi + 6 bytes: 8d b6 00 00 00 00 leal 0(%esi),%esi + 7 bytes: 8d b4 26 00 00 00 00 leal 0(%esi),%esi + + Note that `leal 0(%esi),%esi' has a few different encodings... + + There are faster instructions for certain lengths, that are not true nops. + If you can determine that a register and the condition code is dead (by + scanning forwards for a register that is written before it is read, and + similar for cc) you can use a `incl reg' for a 3 times faster 1 cycle nop... + (From old "NOTES" file to-do list, not really reviewed:) fix relocation types for i860, perhaps by adding a ref pointer to fixS? |