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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/61757] [4.10 Regression] genmodes failure with enable-checking
Date: Mon, 14 Jul 2014 08:56:00 -0000
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X-Bugzilla-Component: tree-optimization
X-Bugzilla-Version: 4.10.0
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61757

--- Comment #23 from Richard Biener <rguenth at gcc dot gnu.org> ---
The first interesting difference is that for

  <bb 14>:
  _793 = ASSERT_EXPR <_73, _73 != 18>;
  switch (_793) <default: <L32>, case 0: <L26>, case 1: <L27>, case 2: <L28>,
case 3: <L29>, case 4 ... 9: <L28>, case 10 ... 11: <L30>, case 12 ... 17:
<L31>>

<L28>:

we record an equality instead of a range:

@@ -17655,13 +17655,7 @@

        BB #19
        EDGE 14->19 14 [14.3%]  (EXECUTABLE)
-       PREDICATE: _73 ge_expr 2
-
-       switch (_73) <default: <L32>, case 0: <L26>, case 1: <L27>, case 2:
<L28>, case 3: <L29>, case 4 ... 9: <L28>, case 10 ... 11: <L30>, case 12 ...
17: <L31>>
-
-       BB #19
-       EDGE 14->19 14 [14.3%]  (EXECUTABLE)
-       PREDICATE: _73 le_expr 9
+       PREDICATE: _73 eq_expr 2

@@ -19815,9 +19806,8 @@
   goto <bb 28>;

 <L28>:
-  _792 = ASSERT_EXPR <_794, _794 >= 2>;
-  _793 = ASSERT_EXPR <_792, _792 <= 9>;
-  _77 = _793 + 4294967288;
+  _792 = ASSERT_EXPR <_793, _793 == 2>;
+  _77 = _792 + 4294967288;
   _78 = _77 > 1;
   _79 = (requirement) _78;
   validate_mode (m_780, 2, 0, 1, 1, _79);

that's of course bogus, happens via find_switch_asserts where for correctly
sorted ci vector we fail to pick up CASE_HIGH of cast 4...9.

We miscompile the loop

          /* Skip labels until the last of the group.  */
          do {
            ++idx;
          } while (idx < n && cbb == ci[idx].bb);

to

   0x0000000000b6a492 <+3074>:  mov    %eax,%r14d
   0x0000000000b6a495 <+3077>:  lea    0x1(%r14),%r15d
=> 0x0000000000b6a499 <+3081>:  cmp    %r15,%rcx
   0x0000000000b6a49c <+3084>:  mov    %r15,%rax
   0x0000000000b6a49f <+3087>:  ja     0xb6a492
<find_assert_locations_1(basic_block, sbitmap)+3074>

that is, we simply increment idx until it is equal to n and then use
CASE_HIGH of the last case label in the vector which is the default
label (and thus does not have CASE_HIGH nor CASE_LOW).

The following testcase derived from that is miscompiled on trunk at -O2:

extern void abort (void);

struct X { void *p; int res; } a[32];

int foo (unsigned i, unsigned n, void *q)
{
  if (i + 1 < n && q == a[i + 1].p)
    {
      do {
          ++i;
      } while (i < n && q == a[i].p);
      --i;
      return a[i].res;
    }
}

int main ()
{
  int x;
  a[0].p = &x;
  a[1].p = &x;
  a[1].res = 1;
  a[2].p = (void *)0;
  a[2].res = 0;
  if (foo (0, 3, &x) != 1)
    abort ();
  return 0;
}