This is a PR where LRA creates a read from an uninitialized stack slot.
That stack slot is supposed to hold the value of the PIC register.
What seems to happen is that we have two passes making different choices:
Choosing alt 0 in insn 143: (0) =x (1) 0 (2) r {sse2_pinsrw}
[...]
Choosing alt 1 in insn 143: (0) x (1) 0 (2) m {sse2_pinsrw}
The second choice causes a constant to be placed in memory, and a new
reference to the PIC register is created. Unfortunately, before the
second pass, we seem to have deleted an insn that stores a reload reg
into the spilled pic register pseudo, because we think we've managed to
inherit the reload reg into all uses. The new use isn't taken into account.
I came up with the following, initially as a hack, but it seems to work
surprisingly well. Bootstrapped and tested on x86_64-linux with -fPIC;
it seems to cure the problem for the testcase (by inspection, it never
crashed on my system).
Since I was worried about code generation differences (adding
unnecessary stores) I've also run it against my collection of .i files.
With -m64 -fPIC, I did not observe any changes. I hadn't looked at
x86_64 pic generation previously, looks like we can use pc-relative
addressing and don't need the pic reg at all usually? With -m32 -fPIC, I
observe differences in 27 out of 4117 source files (taken from the Linux
kernel, gcc, and several other packages). That doesn't seem very
worrying, especially considering that several of these changes turn out
not to be redundant stores, just placing the PIC reg into a different
stack slot.
So... ok everywhere?
Bernd
PR rtl-optimization/69648
* lra-constraints.c (update_ebb_live_info): Don't remove sets of
pic_offset_table_rtx.
Index: lra-constraints.c
===================================================================
--- lra-constraints.c (revision 232689)
+++ lra-constraints.c (working copy)
@@ -5127,8 +5127,10 @@ update_ebb_live_info (rtx_insn *head, rt
curr_id = lra_get_insn_recog_data (curr_insn);
curr_static_id = curr_id->insn_static_data;
remove_p = false;
- if ((set = single_set (curr_insn)) != NULL_RTX && REG_P (SET_DEST
(set))
+ if ((set = single_set (curr_insn)) != NULL_RTX
+ && REG_P (SET_DEST (set))
&& (regno = REGNO (SET_DEST (set))) >= FIRST_PSEUDO_REGISTER
+ && SET_DEST (set) != pic_offset_table_rtx
&& bitmap_bit_p (&check_only_regs, regno)
&& ! bitmap_bit_p (&live_regs, regno))
remove_p = true;
