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[PATCH, PR81844] Fix condition folding in c_parser_omp_for_loop


Hi,

this patch fixes the wrong-code PR81844, where an omp for loop is incorrectly removed by the compiler.


Consider the test-case from the patch.

It contains a omp for condition 'i > 0x7fffffffffffffffULL', where i is of type unsigned long long int.


In c_parser_omp_for_loop, we first have:
...
(gdb) call debug_generic_expr (cond)
<<< Unknown tree: c_maybe_const_expr

  i >>> > 9223372036854775807
...

Then we execute:
...
15030		  cond = c_fully_fold (cond, false, NULL);
...

and we have:
...
(gdb) call debug_generic_expr (cond)
(signed long) i < 0
...

Note that the type of the comparison changed from unsigned to signed.

Subsequently, in c_finish_omp_for we do:
...
              /* 2.5.1.  The comparison in the condition is computed in
                 the type of DECL, otherwise the behavior is undefined.

                 For example:
                 long n; int i;
                 i < n;

                 according to ISO will be evaluated as:
                 (long)i < n;

                 We want to force:
                 i < (int)n;  */
              if (TREE_CODE (op0) == NOP_EXPR
                  && decl == TREE_OPERAND (op0, 0))
                {
                  TREE_OPERAND (cond, 0) = TREE_OPERAND (op0, 0);
                  TREE_OPERAND (cond, 1)
                    = fold_build1_loc (elocus, NOP_EXPR,
                                       TREE_TYPE (decl),
                                   TREE_OPERAND (cond, 1));
                }
...

and we end up with:
...
(gdb) call debug_generic_expr (cond)
i < 0
...
which is always false.


AFAIU, the problem is that the optimization in c_finish_omp_for assumes that the operand type of the condition is the same as in the source code, while c_fully_fold breaks that assumption.


The patch fixes this by folding only the operands of the condition.

Bootstrapped and reg-tested on x86_64.

OK for trunk?

Thanks,
- Tom

Fix condition folding in c_parser_omp_for_loop

2017-08-14  Tom de Vries  <tom@codesourcery.com>

	PR c/81844
	* c-parser.c (c_parser_omp_for_loop): Fix condition folding.

	* testsuite/libgomp.c/pr81805.c: New test.

---
 gcc/c/c-parser.c                      | 19 +++++++++++++++-
 libgomp/testsuite/libgomp.c/pr81805.c | 42 +++++++++++++++++++++++++++++++++++
 2 files changed, 60 insertions(+), 1 deletion(-)

diff --git a/gcc/c/c-parser.c b/gcc/c/c-parser.c
index d018fbc..cba4103 100644
--- a/gcc/c/c-parser.c
+++ b/gcc/c/c-parser.c
@@ -15027,7 +15027,24 @@ c_parser_omp_for_loop (location_t loc, c_parser *parser, enum tree_code code,
 
 	  cond = cond_expr.value;
 	  cond = c_objc_common_truthvalue_conversion (cond_loc, cond);
-	  cond = c_fully_fold (cond, false, NULL);
+	  switch (TREE_CODE (cond))
+	    {
+	    case GT_EXPR:
+	    case GE_EXPR:
+	    case LT_EXPR:
+	    case LE_EXPR:
+	    case NE_EXPR:
+	      {
+		tree op0 = TREE_OPERAND (cond, 0), op1 = TREE_OPERAND (cond, 1);
+		op0 = c_fully_fold (op0, false, NULL);
+		op1 = c_fully_fold (op1, false, NULL);
+		TREE_OPERAND (cond, 0) = op0;
+		TREE_OPERAND (cond, 1) = op1;
+	      }
+	      break;
+	    default:
+	      break;
+	    }
 	  switch (cond_expr.original_code)
 	    {
 	    case GT_EXPR:
diff --git a/libgomp/testsuite/libgomp.c/pr81805.c b/libgomp/testsuite/libgomp.c/pr81805.c
new file mode 100644
index 0000000..fa78b3c
--- /dev/null
+++ b/libgomp/testsuite/libgomp.c/pr81805.c
@@ -0,0 +1,42 @@
+/* { dg-do run } */
+
+extern void abort (void);
+
+#define N 32ULL
+int a[N];
+
+const unsigned long long c = 0x7fffffffffffffffULL;
+
+void
+f2_tpf_static32 (void)
+{
+  unsigned long long i;
+  #pragma omp for
+  for (i = c + N; i > c; i -= 1ULL)
+    a[i - 1ULL - c] -= 4;
+}
+
+__attribute__((noinline, noclone)) int
+test_tpf_static32 (void)
+{
+  int i, j, k;
+  for (i = 0; i < N; i++)
+    a[i] = i - 25;
+
+  f2_tpf_static32 ();
+
+  for (i = 0; i < N; i++)
+    if (a[i] != i - 29)
+      return 1;
+
+  return 0;
+}
+
+int
+main ()
+{
+  if (test_tpf_static32 ())
+    abort ();
+
+  return 0;
+}

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