[Bug tree-optimization/58805] [4.8/4.9 Regression] Inline assembly wrongly optimized out when inside a conditional

[vries at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=58805

--- Comment #14 from vries at gcc dot gnu.org ---
> No it shouldn't.  It should return true if the stmt has side-effects that
> are _not_ explicit in the statement.  This side-effect is explicitely there.

Hmm, a rather deceptive name then, if gimple_stmt_has_side_effects does not in
fact tell whether a gimple statement has side-effects or not.
I wonder if any other use sites are mistaken in the semantics, and what could
be a better name. 

A better name according to your description could be
gimple_stmt_has_implicit_side_effects, but I'm not sure I understand how to
differentiate between implicit and explicit here. Marking an asm with volatile,
is that implicit or explicit, and why?

AFAIU, the name gimple_stmt_has_call_or_volatile_side_effects reflects what the
implementation does.

Anyways, this tentative patch fixes the problem in tail-merge:
...
Author: Tom de Vries <tom@codesourcery.com>
Date:   Mon Oct 21 13:40:28 2013 +0200

        * tree-ssa-tail-merge.c (stmt_local_def): Improve side-effect check.

diff --git a/gcc/testsuite/gcc.dg/pr58805.c b/gcc/testsuite/gcc.dg/pr58805.c
new file mode 100644
index 0000000..6e6eba5
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/pr58805.c
@@ -0,0 +1,23 @@
+/* { dg-do compile } */
+/* { dg-options "-O2 -ftree-tail-merge -fdump-tree-pre" } */
+
+static inline void bar(unsigned long *r)
+{
+  unsigned long t;
+  __asm__ (
+    "movq $42, %[t]\n\t"
+    "movq %[t], %[r]\n\t"
+    : [t] "=&r" (t), [r] "=r" (*r)
+  );
+}
+
+void foo(int n, unsigned long *x, unsigned long *y)
+{
+  if (n == 0)
+    bar(x);
+  else
+    bar(y);
+}
+
+/* { dg-final { scan-tree-dump-times "__asm__" 2 "pre"} } */
+/* { dg-final { cleanup-tree-dump "pre" } } */
diff --git a/gcc/tree-ssa-tail-merge.c b/gcc/tree-ssa-tail-merge.c
index 9094935..0090da6 100644
--- a/gcc/tree-ssa-tail-merge.c
+++ b/gcc/tree-ssa-tail-merge.c
@@ -300,7 +300,8 @@ stmt_local_def (gimple stmt)
   tree val;
   def_operand_p def_p;

-  if (gimple_has_side_effects (stmt))
+  if (gimple_has_side_effects (stmt)
+      || gimple_vdef (stmt) != NULL_TREE)
     return false;

   def_p = SINGLE_SSA_DEF_OPERAND (stmt, SSA_OP_DEF);
...

I'll test this one.