gnu /
gcc /
6b5c98c1c0003bd470a4428bede6c862637a94b8 reg-stack: Fix a -fcompare-debug bug in reg-stack [PR107183]
As the following testcase shows, the swap_rtx_condition function
in reg-stack can result in different code generation between -g and -g0.
The function is doing the changes as it goes, so does analysis and
changes together, which makes it harder to deal with DEBUG_INSNs,
where normally analysis phase ignores them and the later phase
doesn't.
swap_rtx_condition walks instructions two different ways, one is
using next_flags_user function which stops on non-call instructions
that mention the flags register, and the other is a loop on fnstsw
where it stops on instructions mentioning it and tries to find
sahf instruction that uses it (in both cases calls stop it and so
does end of basic block).
Now both of these currently stop on DEBUG_INSNs that mention
the flags register resp. the fnstsw result register.
On success the function recurses on next flags user instruction
if still live and if the recursion failed, reverts the changes
it did too and fails.
If it were just for the next_flags_user case, the fix could be
just not doing
INSN_CODE (insn) = -1;
if (recog_memoized (insn) == -1)
fail = 1;
on DEBUG_INSNs (assuming all changes to those are fine),
swap_rtx_condition_1 just changes one comparison to a different
one. But due to the possibility of fnstsw result being used
in theory before sahf in some DEBUG_INSNs, this patch takes
a different approach. swap_rtx_condition has now a new argument
and two modes. The first mode is when debug_seen is >= 0, in this
case both next_flags_user and the loop for fnstsw -> sahf will
ignore but note DEBUG_INSNs (that mention flags register or fnstsw
result). If no such DEBUG_INSN is found during the whole call
including recursive invocations (so e.g. for -g0 but probably most
often for -g as well), it behaves as before, if it returns true
all the changes are done and nothing further needs to be done later.
If any DEBUG_INSNs are seen along the way, even when returning success
all the changes are reverted, so it just reports that the function
would be successful if DEBUG_INSNs were ignored.
In this case, compare_for_stack_reg needs to call it again in
debug_seen = -1 mode, which tells the function to update everything
including DEBUG_INSNs. For the fnstsw -> sahf case which I hope
will be very rare I just reset the DEBUG_INSNs, I don't really
know how to express it easily otherwise. For the rest
swap_rtx_condition_1 is done even on the DEBUG_INSNs.
2022-11-20 Jakub Jelinek <jakub@redhat.com>
PR target/107183
* reg-stack.cc (next_flags_user): Add DEBUG_SEEN argument.
If >= 0 and a DEBUG_INSN would be otherwise returned, set
DEBUG_SEEN to 1 and ignore it.
(swap_rtx_condition): Add DEBUG_SEEN argument. In >= 0
mode only set DEBUG_SEEN to 1 if problematic DEBUG_ISNSs
were seen and revert all changes on success in that case.
Don't try to recog_memoized DEBUG_INSNs.
(compare_for_stack_reg): Adjust swap_rtx_condition caller.
If it returns true and debug_seen is 1, call swap_rtx_condition
again with debug_seen -1.
* gcc.dg/ubsan/pr107183.c: New test.
2 files changed
