------- Additional Comments From bonzini at gcc dot gnu dot org 2005-08-22
11:01 -------
I'm not confident that using salq in Steven's test case is really a
pessimization. I'll consider a 32-bit target then, and this testcase
char **
VTallocbuf(char **allbuf, unsigned long savelines)
{
return &allbuf[savelines];
}
For i686 we have
movl 8(%esp), %eax
movl 4(%esp), %edx
sall $2, %eax
addl %edx, %eax
instead of
movl 8(%esp), %eax
movl 4(%esp), %edx
leal (%edx,%eax,4), %eax
However even in this case a no-lea code could be feasible:
movl 8(%esp), %eax
sall $2, %eax
addl 4(%esp), %eax
And this is exactly the rtl we have until peephole2, where this peephole splits
the instruction:
;; Don't do logical operations with memory inputs.
(define_peephole2
[(match_scratch:SI 2 "r")
(parallel [(set (match_operand:SI 0 "register_operand" "")
(match_operator:SI 3 "arith_or_logical_operator"
[(match_dup 0)
(match_operand:SI 1 "memory_operand" "")]))
(clobber (reg:CC FLAGS_REG))])]
"! optimize_size && ! TARGET_READ_MODIFY"
[(set (match_dup 2) (match_dup 1))
(parallel [(set (match_dup 0)
(match_op_dup 3 [(match_dup 0) (match_dup 2)]))
(clobber (reg:CC FLAGS_REG))])]
"")
I think that Jan's patch should be conditionalized: if !optimize_size &&
!TARGET_READ_MODIFY, the transformation he removed will be done anyway, and too
late in the game.
Paolo
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23303
