[gcc r11-9494] shrink-wrapping: Fix up prologue block discovery [PR103860]

[gcc r11-9494] shrink-wrapping: Fix up prologue block discovery [PR103860]

[Jakub Jelinek]

https://gcc.gnu.org/g:a4e45a579e274eef09a1f8e7251927c084097322

commit r11-9494-ga4e45a579e274eef09a1f8e7251927c084097322
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Thu Dec 30 14:23:18 2021 +0100

    shrink-wrapping: Fix up prologue block discovery [PR103860]
    
    The following testcase is miscompiled, because a prologue which
    contains subq $8, %rsp instruction is emitted at the start of
    a basic block which contains conditional jump that depends on
    flags register set in an earlier basic block, the prologue instruction
    then clobbers those flags.
    Normally this case is checked by can_get_prologue predicate, but this
    is done only at the start of the loop.  If we update pro later in the
    loop (because some bb shouldn't be duplicated) and then don't push
    anything further into vec and the vec is already empty (this can happen
    when the new pro is already in bb_with bitmask and either has no successors
    (that is the case in the testcase where that bb ends with a trap) or
    all the successors are already in bb_with, then the loop doesn't iterate
    further and can_get_prologue will not be checked.
    
    The following simple patch makes sure we call can_get_prologue even after
    the last former iteration when vec is already empty and only break from
    the loop afterwards (and only if the updating of pro done because of
    !can_get_prologue didn't push anything into vec again).
    
    2021-12-30  Jakub Jelinek  <jakub@redhat.com>
    
            PR rtl-optimization/103860
            * shrink-wrap.c (try_shrink_wrapping): Make sure can_get_prologue is
            called on pro even if nothing further is pushed into vec.
    
            * gcc.dg/pr103860.c: New test.
    
    (cherry picked from commit 1820137ba624d7eb2004a10f9632498b6bc1696a)

Diff:
---
 gcc/shrink-wrap.c               |  5 ++++-
 gcc/testsuite/gcc.dg/pr103860.c | 31 +++++++++++++++++++++++++++++++
 2 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/gcc/shrink-wrap.c b/gcc/shrink-wrap.c
index 87ea3b58a02..7bac367a39b 100644
--- a/gcc/shrink-wrap.c
+++ b/gcc/shrink-wrap.c
@@ -781,7 +781,7 @@ try_shrink_wrapping (edge *entry_edge, rtx_insn *prologue_seq)
   unsigned max_grow_size = get_uncond_jump_length ();
   max_grow_size *= param_max_grow_copy_bb_insns;
 
-  while (!vec.is_empty () && pro != entry)
+  while (pro != entry)
     {
       while (pro != entry && !can_get_prologue (pro, prologue_clobbered))
 	{
@@ -791,6 +791,9 @@ try_shrink_wrapping (edge *entry_edge, rtx_insn *prologue_seq)
 	    vec.quick_push (pro);
 	}
 
+      if (vec.is_empty ())
+	break;
+
       basic_block bb = vec.pop ();
       if (!can_dup_for_shrink_wrapping (bb, pro, max_grow_size))
 	while (!dominated_by_p (CDI_DOMINATORS, bb, pro))
diff --git a/gcc/testsuite/gcc.dg/pr103860.c b/gcc/testsuite/gcc.dg/pr103860.c
new file mode 100644
index 00000000000..15eee558083
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr103860.c
@@ -0,0 +1,31 @@
+/* PR rtl-optimization/103860 */
+/* { dg-do run } */
+/* { dg-options "-O3" } */
+/* { dg-additional-options "-fPIC" { target fpic } } */
+
+static int d, *e;
+int f;
+
+__attribute__((noinline)) signed char
+foo (signed char b, signed char c)
+{
+  return b + c;
+}
+
+int
+main ()
+{
+  signed char l;
+  for (l = -1; l; l = foo (l, 1))
+    {
+      while (d < 0)
+	;
+      if (d > 0)
+	{
+	  f = 0;
+	  *e = 0;
+	}
+    }
+  d = 0;
+  return 0;
+}