On Thu, Dec 1, 2011 at 11:45 PM, Richard Guenther <richard.guent...@gmail.com> wrote:
> Well, it's not that easy if you still want to properly do redundant expression
> removal on global registers.
Yes, it might be complicate to make PRE fully aware of global register.
I also found comments in is_gimple_reg which says gcc does not do
much optimization with register variable at the tree level for now.
Back to this issue, I think it can be fixed by following way without hurting
redundancy elimination on global register variables:
After insert() being called in pre, in function eliminate() we can check for
single assignment statement from global register variable to ssa_name.
If it is the case, we can just skip the elimination operation.
In this way:
1, normal redundancy elimination on global registers will not be hurt,
since sccvn and pre has already detected the true elimination chances
and they will be eliminated afterward in function eliminate;
2, the inserted statements(including PHIs) for global register variables
will not be marked as NECESSARY in function eliminate and will be
deleted in remove_dead_inserted_code;
I attached an example which can illustrates that the normal redundancy does
get eliminated.
I will send a patch for review if it worth a discuss. So what do you think?
Thanks
--
Best Regards.
/* { dg-do compile } */
/* { dg-options "-O2 -fdump-tree-pre-stats" } */
register int data_0 asm("r4");
register int data_3 asm("r5");
int motion_test1(int data, int v)
{
int i;
int t, u;
if (data)
i = data_0 + data_3;
else {
v = 2;
i = 5;
}
t = data_0 + data_3;
u = i;
return v * t * u;
}
/* We should eliminate one computation of data_0 + data_3 along the
main path. We cannot re-associate v * t * u due to undefined
signed overflow so we do not eliminate one computation of v * i along
the main path. */
/* { dg-final { scan-tree-dump-times "Eliminated: 2" 1 "pre" { xfail *-*-* } } } */
/* { dg-final { scan-tree-dump-times "Eliminated: 1" 1 "pre" } } */
/* { dg-final { cleanup-tree-dump "pre" } } */
ssa-pre-2.c.093t.crited
Description: Binary data
ssa-pre-2.c.094t.pre.orig
Description: Binary data
;; Function motion_test1 (motion_test1, funcdef_no=0, decl_uid=4055, cgraph_uid=0)
Points-to analysis
Constraints:
ANYTHING = &ANYTHING
READONLY = &READONLY
ESCAPED = *ESCAPED
ESCAPED = ESCAPED + UNKNOWN
*ESCAPED = NONLOCAL
NONLOCAL = &NONLOCAL
NONLOCAL = &ESCAPED
INTEGER = &ANYTHING
data = &NONLOCAL
v = &NONLOCAL
*r4 = NONLOCAL
data_0.0_4 = *r4
*r5 = NONLOCAL
data_3.1_5 = *r5
i_6 = data_0.0_4
i_6 = data_3.1_5
v_1 = v
v_1 = &NONLOCAL
i_2 = i_6
i_2 = &NONLOCAL
data_0.0_10 = *r4
data_3.1_11 = *r5
t_12 = data_0.0_10
t_12 = data_3.1_11
D.4935_14 = v_1
D.4935_14 = t_12
D.4934_15 = i_2
D.4934_15 = D.4935_14
ESCAPED = D.4934_15
Collapsing static cycles and doing variable substitution
Building predecessor graph
Detecting pointer and location equivalences
Rewriting constraints and unifying variables
Uniting pointer but not location equivalent variables
Finding indirect cycles
Solving graph
Points-to sets
ANYTHING = { ANYTHING }
READONLY = { READONLY }
ESCAPED = { ESCAPED NONLOCAL }
NONLOCAL = { ESCAPED NONLOCAL } same as *r4
STOREDANYTHING = { }
INTEGER = { ANYTHING }
data = { NONLOCAL }
v = { NONLOCAL } same as data
data_0.0_4 = { ESCAPED NONLOCAL } same as *r4
*r4 = { ESCAPED NONLOCAL }
data_3.1_5 = { ESCAPED NONLOCAL } same as *r4
*r5 = { ESCAPED NONLOCAL } same as *r4
i_6 = { ESCAPED NONLOCAL } same as *r4
v_1 = { NONLOCAL } same as data
i_2 = { ESCAPED NONLOCAL } same as *r4
data_0.0_10 = { ESCAPED NONLOCAL } same as *r4
data_3.1_11 = { ESCAPED NONLOCAL } same as *r4
t_12 = { ESCAPED NONLOCAL } same as *r4
D.4935_14 = { ESCAPED NONLOCAL } same as *r4
D.4934_15 = { ESCAPED NONLOCAL } same as *r4
Alias information for motion_test1
Aliased symbols
.MEM, UID D.4937, void, is global, default def: .MEM_16(D)
data_0, UID D.4051, int, is global
data_3, UID D.4052, int, is global
Call clobber information
ESCAPED, points-to non-local, points-to vars: { }
Flow-insensitive points-to information
;; 1 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2 5 3 4
;; 2 succs { 3 5 }
;; 5 succs { 4 }
;; 3 succs { 4 }
;; 4 succs { 1 }
Could not find SSA_NAME representative for expression:{mult_expr,i_6,2}
Created SSA_NAME representative pretmp.5_17 for expression:{mult_expr,i_6,2}
Could not find SSA_NAME representative for expression:{mult_expr,i_6,v_7(D)}
Created SSA_NAME representative pretmp.5_18 for expression:{mult_expr,i_6,v_7(D)}
Symbols to be put in SSA form
{ .MEM }
Incremental SSA update started at block: 0
Number of blocks in CFG: 6
Number of blocks to update: 5 ( 83%)
motion_test1 (int data, int v)
{
int prephitmp.6;
int pretmp.5;
int t;
int i;
int D.4935;
int D.4934;
int data_3.1;
int data_0.0;
<bb 2>:
if (data_3(D) != 0)
goto <bb 3>;
else
goto <bb 5>;
<bb 5>:
pretmp.5_19 = data_0;
pretmp.5_21 = data_3;
pretmp.5_23 = pretmp.5_19 + pretmp.5_21;
goto <bb 4>;
<bb 3>:
data_0.0_4 = data_0;
data_3.1_5 = data_3;
i_6 = data_0.0_4 + data_3.1_5;
<bb 4>:
# v_1 = PHI <v_7(D)(3), 2(5)>
# i_2 = PHI <i_6(3), 5(5)>
# prephitmp.6_24 = PHI <i_6(3), pretmp.5_23(5)>
data_0.0_10 = data_0;
data_3.1_11 = data_3;
t_12 = prephitmp.6_24;
D.4935_14 = v_1 * t_12;
D.4934_15 = i_2 * D.4935_14;
return D.4934_15;
}
