Hi Paul,
On i386 (and X86_64) RTL for insn X is generated with a "(clobber reg:CC
FLAGS_REG)" instead of indicating exactly what is written on flags regs. I
don't know if this could be different (as you suggested).
Maybe the idea is combine "operation insns" and "test insns" later, but
combine is not able to do it on some cases.
here it is the insns generated by the i386 backend just before combine pass:
(insn 7 6 8 2 (parallel [
(set (reg:SI 61 [ a.2 ])
(plus:SI (reg:SI 64 [ a ])
(reg:SI 63 [ b ])))
(clobber (reg:CC 17 flags))
]) ../i386_tests/test_and.c:7 252 {*addsi_1}
(expr_list:REG_DEAD (reg:SI 64 [ a ])
(expr_list:REG_DEAD (reg:SI 63 [ b ])
(expr_list:REG_UNUSED (reg:CC 17 flags)
(expr_list:REG_EQUAL (plus:SI (mem/c/i:SI (symbol_ref:SI ("a")
<var_decl 0x2aba19b5f000 a>) [2 a+0 S4 A32])
(mem/c/i:SI (symbol_ref:SI ("b") <var_decl
0x2aba19b5f0a0 b>) [2 b+0 S4 A32]))
(nil))))))
(insn 8 7 9 2 (set (mem/c/i:SI (symbol_ref:SI ("a") <var_decl 0x2aba19b5f000
a>) [2 a+0 S4 A32])
(reg:SI 61 [ a.2 ])) ../i386_tests/test_and.c:7 64 {*movsi_internal}
(nil))
(insn 9 8 10 2 (set (reg:CCZ 17 flags)
(compare:CCZ (reg:SI 61 [ a.2 ])
(const_int 0 [0]))) ../i386_tests/test_and.c:9 2 {*cmpsi_ccno_1}
(expr_list:REG_DEAD (reg:SI 61 [ a.2 ])
(nil)))
Em 27/04/2011 16:20, Paul Koning < paul_kon...@dell.com > escreveu:
On Apr 27, 2011, at 3:15 PM, cirrus75 wrote:
>
> Hello Ian,
>
> One example is:
>
> insn X : "REG_X = "
> insn X+1 : "MEM(addr) = REG_X"
> insn X+2 : "REGY:CCmode compare(REG_X, const_int 0)"
>
> generated by C code (already posted by me some weeks ago):
> ------
>
> int a, b, c, d;
>
> int foo()
> {
> a += b;
>
> if(a)
> c = d;
> }
>
> Insns X+2 and X can usually be combined because arithmetic operation
> usually sets condition codes.
I haven't gotten into this much yet, so at the risk of showing off confusion...
I thought that the CCmode stuff allows this to work right without new changes,
given that the expressions that make up the RTL are written as (parallel ...)
which set both the output reg and the CCmode reg (based on the expression
value). So the rtl for the first insn would have that compare as part of its
parallel...construct, the second insn (presumably in your example) doesn't
affect the condition codes register, and the third insn should then be deleted
since it's redundant.
Doesn't it work like that? Am I confused about the right way?
paul
