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* [Bug tree-optimization/57361] New: Remove self memory assignment
@ 2013-05-21 19:10 glisse at gcc dot gnu.org
2013-05-22 8:35 ` [Bug tree-optimization/57361] " rguenth at gcc dot gnu.org
2013-06-13 19:47 ` glisse at gcc dot gnu.org
0 siblings, 2 replies; 3+ messages in thread
From: glisse at gcc dot gnu.org @ 2013-05-21 19:10 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57361
Bug ID: 57361
Summary: Remove self memory assignment
Product: gcc
Version: 4.9.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: glisse at gcc dot gnu.org
Hello,
as noticed in PR 57303, tree level optimizations don't remove self memory
assignments:
typedef struct A{long l,m,n,o,p;} A;
void f(A*x){
*x=*x;
}
compiled with -O3 still has the assignment in the .optimized dump.
I suggested removing statements where rhs is tcc_reference and
operand_equal_p(lhs,rhs), but Richard warns: "The obvious place would be dead
store elimination. But beware that the above, if not _literally_ being *p = *p
can be changing the effective type of the memory location and thus might be not
dead in terms of type-based aliasing rules (which basically means that we need
to do sth more clever than just removing the store)."
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/57361] Remove self memory assignment
2013-05-21 19:10 [Bug tree-optimization/57361] New: Remove self memory assignment glisse at gcc dot gnu.org
@ 2013-05-22 8:35 ` rguenth at gcc dot gnu.org
2013-06-13 19:47 ` glisse at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: rguenth at gcc dot gnu.org @ 2013-05-22 8:35 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57361
Richard Biener <rguenth at gcc dot gnu.org> changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Last reconfirmed| |2013-05-22
Ever confirmed|0 |1
--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
Assignment expansion makes this optimization:
/* Optimize away no-op moves without side-effects. */
if (operand_equal_p (to, from, 0))
return;
The operand_equal_p implementation looks safe at the moment (even in terms
of type-based aliasing rules).
I'd have done a more aggressive optimization in DSE, doing
if (stmt_kills_ref_p (stmt, gimple_assign_rhs1 (stmt)))
... must be a self-assignment ...
which would definitely need extra care for self-assignments that change
the effective type of a memory location. Though I have a hard time
creating a testcase that would exhibit sth like
MEM[(struct X *)q_1] = MEM[(struct Y *)q_1];
which would change the dynamic type of *q_1 to X from Y.
^ permalink raw reply [flat|nested] 3+ messages in thread
* [Bug tree-optimization/57361] Remove self memory assignment
2013-05-21 19:10 [Bug tree-optimization/57361] New: Remove self memory assignment glisse at gcc dot gnu.org
2013-05-22 8:35 ` [Bug tree-optimization/57361] " rguenth at gcc dot gnu.org
@ 2013-06-13 19:47 ` glisse at gcc dot gnu.org
1 sibling, 0 replies; 3+ messages in thread
From: glisse at gcc dot gnu.org @ 2013-06-13 19:47 UTC (permalink / raw)
To: gcc-bugs
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57361
--- Comment #2 from Marc Glisse <glisse at gcc dot gnu.org> ---
I'll let Richard decide if this should stay open for the more aggressive
optimization or not.
r200034 | glisse | 2013-06-12 19:56:30 +0200 (Wed, 12 Jun 2013) | 10 lines
2013-06-12 Marc Glisse <marc.glisse@inria.fr>
PR tree-optimization/57361
gcc/
* tree-ssa-dse.c (dse_possible_dead_store_p): Handle self-assignment.
gcc/testsuite/
* gcc.dg/tree-ssa/pr57361.c: New file.
^ permalink raw reply [flat|nested] 3+ messages in thread
end of thread, other threads:[~2013-06-13 19:47 UTC | newest]
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2013-05-21 19:10 [Bug tree-optimization/57361] New: Remove self memory assignment glisse at gcc dot gnu.org
2013-05-22 8:35 ` [Bug tree-optimization/57361] " rguenth at gcc dot gnu.org
2013-06-13 19:47 ` glisse at gcc dot gnu.org
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