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* [Bug target/25008] New: problems with "S" constraint in asms
@ 2005-11-23 21:55 wilson at gcc dot gnu dot org
2006-02-26 19:09 ` [Bug target/25008] " pinskia at gcc dot gnu dot org
0 siblings, 1 reply; 2+ messages in thread
From: wilson at gcc dot gnu dot org @ 2005-11-23 21:55 UTC (permalink / raw)
To: gcc-bugs
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The following testcase compiles with gcc-4.0, but generates an error with
optimization with gcc-4.1.
int *
sub (int *i, int k)
{
int j;
for (j = 0; j < 16; j++)
{
asm volatile ("foo %0" : "=S" (*i));
i += k;
}
return i;
}
aretha$ ./xgcc -B./ -O2 -S -da tmp.c
tmp.c: In function sub:
tmp.c:7: error: asm operand requires impossible reload
This was broken by my patch that defined EXTRA_MEMORY_CONSTRAINT.
http://gcc.gnu.org/ml/gcc-patches/2005-08/msg00714.html
I didn't have a testcase for that at the time, but now that I've run into
trouble, I went to the effort of creating of creating one. It required adding
two letters, and deleting one. The following testcase fails when compiled
without optimization with gcc-4.0.
int *
sub (int *i, int k)
{
int j;
for (j = 0; j < 16; j++)
{
asm volatile ("foo %0" : : "S" (*i));
i += k;
}
return i;
}
aretha$ ./xgcc -B./ -S -da tmp2.c
tmp2.c: In function sub:
tmp2.c:7: error: impossible constraint in asm
The reason why defining EXTRA_MEMORY_CONSTRAINT fails for the first example is
because asm_operand_ok has code that says any memory operand is OK if
EXTRA_MEMORY_CONSTRAINT is true because it can be reloaded to fit. This is
true in theory. Unfortunately, reload doesn't know how to fix a MEM with a
POST_MODIFY address. It fixes all MEMs that didn't quite match a MEM
constraint where an offsettable address is OK by reloading the address.
else if (goal_alternative_matched[i] == -1
&& goal_alternative_offmemok[i]
&& MEM_P (recog_data.operand[i]))
{
operand_reloadnum[i]
= push_reload (XEXP (recog_data.operand[i], 0), NULL_RTX,...
So if we have an operand (MEM (POST_MODIFY ...)) it is fixed by emitting an
insn (set (reg) (POST_MODIFY ...)) which fails to be recognized triggering the
error. find_reloads_address knows how to fix this, but of course did not do
anything because this address is accepted by GO_IF_LEGITIMATE_ADDRESS.
The second example fails without EXTRA_MEMORY_CONSTRAINT defined because of
parse_input_constraint in stmt.c. If EXTRA_CONSTRAINT_STR is not defined, then
it decides that no operand is acceptable. When I posted the earlier patch, I
mentioned that it looked like we had a misplaced #endif, since the default here
should be to just accept all operands if we can't handle the constraint letter.
Unfortunately, taking a second look, I see that a parse_input_constraint change
doesn't work, because gimplify_asm_expr gives me the MEM operand I need only if
!allows_reg.
So it looks like I have to try to fix reload if I want this to work.
--
Summary: problems with "S" constraint in asms
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: wilson at gcc dot gnu dot org
GCC target triplet: ia64-*-*
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25008
^ permalink raw reply [flat|nested] 2+ messages in thread
* [Bug target/25008] problems with "S" constraint in asms
2005-11-23 21:55 [Bug target/25008] New: problems with "S" constraint in asms wilson at gcc dot gnu dot org
@ 2006-02-26 19:09 ` pinskia at gcc dot gnu dot org
0 siblings, 0 replies; 2+ messages in thread
From: pinskia at gcc dot gnu dot org @ 2006-02-26 19:09 UTC (permalink / raw)
To: gcc-bugs
------- Comment #1 from pinskia at gcc dot gnu dot org 2006-02-26 18:47 -------
Confirmed.
--
pinskia at gcc dot gnu dot org changed:
What |Removed |Added
----------------------------------------------------------------------------
Status|UNCONFIRMED |NEW
Ever Confirmed|0 |1
Last reconfirmed|0000-00-00 00:00:00 |2006-02-26 18:47:23
date| |
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=25008
^ permalink raw reply [flat|nested] 2+ messages in thread
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2005-11-23 21:55 [Bug target/25008] New: problems with "S" constraint in asms wilson at gcc dot gnu dot org
2006-02-26 19:09 ` [Bug target/25008] " pinskia at gcc dot gnu dot org
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