Hi,
PR48308 is a case where on the ARM port we incorrectly delete a call
to strcmp assuming that this is dead. However the problem starts early
enough in combine where when try_combine is given i2 of the form
(parallel [(set (reg:CC X) (compare:CC OP (const_int 0)))
(set Y OP)])
and i1 is NULL
it transforms this to :
(set Y OP)
and change I2 to be
(set (reg:CC X) (compare:CC Y (const_int 0)))
but in the snippet of code that changes i1 and i2 we don't seem to
update LOG_LINKS .
We then go and check if i1_feeds_into_i2 and that check relies on the
LOG_LINKS being up-to-date. We find that i2 can be combined with i3
*but* we've then gone and made a transformation that results in Y
being used but miss out emitting the set of Y.
The attached patch appears to fix the problem for the reduced testcase
and reduced^2 testcase attached to PR48308.
Unfortunately this doesn't fix the testcase from PR50313 which
prima-facie was a dup of this bug which I'm still investigating. This
has survived bootstrap on x86_64 and is running tests there ( though
based on a quick reading of the x86 backend I couldn't find any
parallels of that form) and is still undergoing a full bootstrap run
on an ARM board.
Thoughts ?
cheers
Ramana
diff --git a/gcc/combine.c b/gcc/combine.c
index 4178870..f6b8769 100644
--- a/gcc/combine.c
+++ b/gcc/combine.c
@@ -2865,6 +2865,8 @@ try_combine (rtx i3, rtx i2, rtx i1, rtx i0, int
*new_direct_jump_p,
SUBST (PATTERN (i2), XVECEXP (PATTERN (i2), 0, 0));
SUBST (XEXP (SET_SRC (PATTERN (i2)), 0),
SET_DEST (PATTERN (i1)));
+ LOG_LINKS (i2) = alloc_insn_link (i1, LOG_LINKS (i2));
+
}
}
#endif
