>> +/* Return true if the use is defined by a gcov counter var.
>> + It is used to check if an iv candidate is generated for
>> + profiling. For profile generated ssa name, we should not
>> + use it as IV because gcov counter may have data-race for
>> + multithread program, it could involve tricky bug to use
>> + such ssa var in IVOPT.
>> +
>
> Add the snippets describing how ralloc introduces a second gcov
> counter load and asynchronous update of it from another thread leading
> to bogus trip count.
>
> Also mention that setting volatile flag on gcov counter accesses may
> greatly degrade profile-gen performance.
>
Comments added.
>> + if (!is_gimple_assign (stmt))
>> + return false;
>> +
>> + rhs = gimple_assign_rhs1 (stmt);
>> + if (TREE_CODE (rhs) != ARRAY_REF)
>> + return false;
>> +
>> + decl = TREE_OPERAND (rhs, 0);
>> + if (TREE_CODE (decl) != VAR_DECL)
>> + return false;
>
>
>
> Also check TREE_STATIC and DECL_ARTIFICIAL flag.
>
>
> David
>
Check added. Add DECL_ARTIFICIAL setting in build_var() in coverage.c.
The updated patch is attached.
Thanks,
Wei.
2014-02-11 Wei Mi <w...@google.com>
* tree-ssa-loop-ivopts.c (defined_by_gcov_counter): New.
(contains_abnormal_ssa_name_p): Add defined_by_gcov_counter
check for ssa name.
* coverage.c (build_var): Set DECL_ARTIFICIAL(gcov var decl)
to be 1.
* testsuite/gcc.dg/profile-generate-4.c: New.
Index: testsuite/gcc.dg/profile-generate-4.c
===================================================================
--- testsuite/gcc.dg/profile-generate-4.c (revision 0)
+++ testsuite/gcc.dg/profile-generate-4.c (revision 0)
@@ -0,0 +1,21 @@
+/* { dg-do compile { target { i?86-*-* x86_64-*-* } } } */
+/* { dg-options "-O2 -fprofile-generate -fno-tree-loop-im
-fdump-tree-ivopts-details-blocks" } */
+
+/* Because gcov counter related var has data race for multithread program,
+ compiler should prevent them from affecting program correctness. So
+ PROF_edge_counter variable should not be used as induction variable, or
+ else IVOPT may use such variable to compute loop boundary. */
+
+void *ptr;
+int N;
+
+void foo(void *t) {
+ int i;
+ for (i = 0; i < N; i++) {
+ t = *(void **)t;
+ }
+ ptr = t;
+}
+
+/* { dg-final { scan-tree-dump-times "ssa name PROF_edge_counter" 0 "ivopts"}
} */
+/* { dg-final { cleanup-tree-dump "ivopts" } } */
Index: coverage.c
===================================================================
--- coverage.c (revision 207019)
+++ coverage.c (working copy)
@@ -1485,6 +1485,7 @@ build_var (tree fn_decl, tree type, int
TREE_STATIC (var) = 1;
TREE_ADDRESSABLE (var) = 1;
DECL_ALIGN (var) = TYPE_ALIGN (type);
+ DECL_ARTIFICIAL (var) = 1;
return var;
}
Index: tree-ssa-loop-ivopts.c
===================================================================
--- tree-ssa-loop-ivopts.c (revision 207019)
+++ tree-ssa-loop-ivopts.c (working copy)
@@ -705,6 +705,115 @@ idx_contains_abnormal_ssa_name_p (tree b
return !abnormal_ssa_name_p (*index);
}
+/* Return true if the use is defined by a gcov counter var.
+ It is used to check if an iv candidate is generated for
+ profiling. For profile generated ssa name, we should not
+ use it as IV because gcov counter may have data-race for
+ multithread program, it is compiler's responsibility to
+ avoid connecting profile counter related vars with program
+ correctness.
+
+ Without the check, the following bug could happen in
+ following case:
+ * original loop
+
+ int i;
+ for (i = 0; i < N; i++) {
+ t = *(void **)t;
+ }
+
+ * after profile-gen and IVOPT, loop condition is replaced and
+ pretmp_1 is involved in loop boundary computation.
+
+ pretmp_1 = __gcov0.foo[0];
+ _22 = pretmp_1 + 1;
+ ...
+ _31 = (unsigned long) pretmp_1;
+ _32 = _30 + _31;
+ _33 = _32 + 2;
+ label:
+ ivtmp.8_9 = PHI <ivtmp.8_5(5), ivtmp.8_2(3)>
+ PROF_edge_counter_10 = (long int) ivtmp.8_9;
+ __gcov0.foo[0] = PROF_edge_counter_10;
+ ...
+ ivtmp.8_5 = ivtmp.8_9 + 1;
+ if (ivtmp.8_5 != _33)
+ goto label
+
+ * after register allocation, pretmp_1 may be marked as REG_EQUIV in IRA
+ with __gcov0.foo[0] and some references are replaced by __gcov0.foo in
LRA.
+
+ _22 = __gcov0.foo[0] + 1;
+ ...
+ _31 = (unsigned long) __gcov0.foo[0];
+ _32 = _30 + _31;
+ _33 = _32 + 2;
+ label:
+ ....
+
+ * Bug happens when __gcov0.foo[0] is updated asynchronously by other thread
+ between the above __gcov0.foo[0] references statements.
+
+ We don't choose to mark gcov counter as volatile because it may greatly
+ degrade profile-gen performance.
+
+ To limit patterns to be checked, we list the possible cases
+ here:
+ Before PRE, the ssa name used to set __gcov counter is as
+ follows:
+ for () {
+ PROF_edge_counter_1 = __gcov.foo[i];
+ PROF_edge_counter_2 = PROF_edge_counter_1 + 1;
+ __gcov.foo[i] = PROF_edge_counter_2;
+ }
+ If PRE works, the loop may be transformed to:
+ pretmp_1 = __gcov.foo[i];
+ for () {
+ prephitmp_1 = PHI (PROF_edge_counter_2, pretmp_1);
+ PROF_edge_counter_1 = prephitmp_1;
+ PROF_edge_counter_2 = PROF_edge_counter_1 + 1;
+ __gcov.foo[i] = PROF_edge_counter_2;
+ }
+ So there are two cases:
+ case1: If PRE doesn't work, PROF_edge_counter_1 and PROF_edge_counter_2
+ are neither induction variables candidates. We don't have to worry
+ about this case.
+ case2: If PRE works, the iv candidate base of PROF_edge_counter_1 and
+ PROF_edge_counter_2 are pretmp_1 or pretmp_1 + 1. pretmp_1 is defined
+ by __gcov var.
+
+ So this func only has to check case2. For a ssa name which is an iv
+ candidate, check its base USE and see if it is defined by __gcov var.
+ Returning true means the ssa name is generated for profiling. */
+
+bool
+defined_by_gcov_counter (tree use)
+{
+ gimple stmt;
+ tree rhs, decl;
+ const char *name;
+
+ stmt = SSA_NAME_DEF_STMT (use);
+ if (!is_gimple_assign (stmt))
+ return false;
+
+ rhs = gimple_assign_rhs1 (stmt);
+ if (TREE_CODE (rhs) != ARRAY_REF)
+ return false;
+
+ decl = TREE_OPERAND (rhs, 0);
+ if (TREE_CODE (decl) != VAR_DECL
+ || !TREE_STATIC (decl)
+ || !DECL_ARTIFICIAL (decl))
+ return false;
+
+ name = IDENTIFIER_POINTER (DECL_NAME (decl));
+ if (strncmp (name, "__gcov", 6))
+ return false;
+
+ return true;
+}
+
/* Returns true if EXPR contains a ssa name that occurs in an
abnormal phi node. */
@@ -721,7 +830,8 @@ contains_abnormal_ssa_name_p (tree expr)
codeclass = TREE_CODE_CLASS (code);
if (code == SSA_NAME)
- return SSA_NAME_OCCURS_IN_ABNORMAL_PHI (expr) != 0;
+ return SSA_NAME_OCCURS_IN_ABNORMAL_PHI (expr) != 0
+ || defined_by_gcov_counter (expr);
if (code == INTEGER_CST
|| is_gimple_min_invariant (expr))
