[gcc r8-10879] expand: Fix up find_bb_boundaries [PR98331]

[gcc r8-10879] expand: Fix up find_bb_boundaries [PR98331]

[Jakub Jelinek]

https://gcc.gnu.org/g:8cc0976478d298d587969a48de0e168f7f09fa36

commit r8-10879-g8cc0976478d298d587969a48de0e168f7f09fa36
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Fri Jan 29 10:30:09 2021 +0100

    expand: Fix up find_bb_boundaries [PR98331]
    
    When expansion emits some control flow insns etc. inside of a former GIMPLE
    basic block, find_bb_boundaries needs to split it into multiple basic
    blocks.
    The code needs to ignore debug insns in decisions how many splits to do or
    where in between some non-debug insns the split should be done, but it can
    decide where to put debug insns if they can be kept and otherwise throws
    them away (they can't stay outside of basic blocks).
    On the following testcase, we end up in the bb from expander with
    control flow insn
    debug insns
    barrier
    some other insn
    (the some other insn is effectively dead after __builtin_unreachable and
    we'll optimize that out later).
    Without debug insns, we'd do the split when encountering some other insn
    and split after PREV_INSN (some other insn), i.e. after barrier (and the
    splitting code then moves the barrier in between basic blocks).
    But if there are debug insns, we actually split before the first debug insn
    that appeared after the control flow insn, so after control flow insn,
    and get a basic block that starts with debug insns and then has a barrier
    in the middle that nothing moves it out of the bb.  This leads to ICEs and
    even if it wouldn't, different behavior from -g0.
    The reason for treating debug insns that way is a different case, e.g.
    control flow insn
    debug insns
    some other insn
    or even
    control flow insn
    barrier
    debug insns
    some other insn
    where splitting before the first such debug insn allows us to keep them
    while otherwise we would have to drop them on the floor, and in those
    situations we behave the same with -g0 and -g.
    
    So, the following patch fixes it by resetting debug_insn not just when
    splitting the blocks (it is set only after seeing a control flow insn and
    before splitting for it if needed), but also when seeing a barrier,
    which effectively means we always throw away debug insns after a control
    flow insn and before following barrier if any, but there is no way around
    that, control flow insn must be the last in the bb (BB_END) and BARRIER
    after it, debug insns aren't allowed outside of bb.
    We still handle the other cases fine (when there is no barrier or when
    debug insns appear only after the barrier).
    
    2021-01-29  Jakub Jelinek  <jakub@redhat.com>
    
            PR debug/98331
            * cfgbuild.c (find_bb_boundaries): Reset debug_insn when seeing
            a BARRIER.
    
            * gcc.dg/pr98331.c: New test.
    
    (cherry picked from commit ea0e1eaa30f42e108f6c716745347cc1dcfdc475)

Diff:
---
 gcc/cfgbuild.c                 |  1 +
 gcc/testsuite/gcc.dg/pr98331.c | 18 ++++++++++++++++++
 2 files changed, 19 insertions(+)

diff --git a/gcc/cfgbuild.c b/gcc/cfgbuild.c
index 1d10298818f..30cbb50406b 100644
--- a/gcc/cfgbuild.c
+++ b/gcc/cfgbuild.c
@@ -545,6 +545,7 @@ find_bb_boundaries (basic_block bb)
 	     if the barrier were preceded by a control_flow_insn_p insn.  */
 	  if (!flow_transfer_insn)
 	    flow_transfer_insn = prev_nonnote_nondebug_insn_bb (insn);
+	  debug_insn = NULL;
 	}
 
       if (control_flow_insn_p (insn))
diff --git a/gcc/testsuite/gcc.dg/pr98331.c b/gcc/testsuite/gcc.dg/pr98331.c
new file mode 100644
index 00000000000..951b7586a28
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr98331.c
@@ -0,0 +1,18 @@
+/* PR debug/98331 */
+/* { dg-do compile } */
+/* { dg-options "-g -O2 -fcompare-debug" } */
+/* { dg-additional-options "-march=x86-64" { target { i?86-*-* x86_64-*-* } } } */
+
+void bar (const char *);
+unsigned long long x;
+
+void
+foo (void)
+{
+  int a = 1;
+  bar ("foo");
+  int b = 2;
+  __atomic_fetch_add (&x, 1, 0);
+  int c = 3;
+  __builtin_unreachable ();
+}