On 04/24/2012 11:56 PM, Bin.Cheng wrote:
Hi,
In scan_one_insn, gcc checks whether an insn loads a parameter from
its stack slot, and then
record the fact by decrease the memory cost.
What I do not understand is the check condition like below checks the
REG_EQUIV note, rather than
checking memory access using stack pointer directly.
if (set != 0&& REG_P (SET_DEST (set))&& MEM_P (SET_SRC (set))
&& (note = find_reg_note (insn, REG_EQUIV, NULL_RTX)) != NULL_RTX
&& ((MEM_P (XEXP (note, 0)))
|| (CONSTANT_P (XEXP (note, 0))
&& targetm.legitimate_constant_p (GET_MODE (SET_DEST (set)),
XEXP (note, 0))
&& REG_N_SETS (REGNO (SET_DEST (set))) == 1)))
So what's the connection between REG_EQUIV and stack slot for
parameters? I noticed from
below dumps of IRA pass, seems the annotated insn is not load
parameter from stack, but it is
treated as the check condition. Why?
Dump of IRA:
(insn 121 118 122 6 (set (reg/f:SI 252 [ l_curve ])
(mem/f/c:SI (reg/f:SI 230) [7 l_curve+0 S4 A32]))
bezier01/bmark_lite.c:246 186 {*thumb1_movsi_insn}
(expr_list:REG_EQUIV (mem/f/c:SI (reg/f:SI 230) [7 l_curve+0 S4 A32])
(expr_list:REG_EQUAL (mem/f/c:SI (symbol_ref:SI ("l_curve")
[flags 0x80]<var_decl 0xb768d0c0 l_curve>) [7 l_curve+0 S4 A32])
(nil))))
I am not sure if I missed something important, please help. Thanks very much.
Reload uses equivalent memory (not an allocated stack slot) if the
pseudo did not get a hard registers. Therefore the equivalent insns for
the pseudo (they can be more than one) are not necessary and removed and
that decreases memory cost. Many cases contain loading parameters from
the stack. But there are a lot of other cases too.
