Re: [PR tree-optimization/84224] do not ICE on malformed allocas

[Aldy Hernandez]

On 02/07/2018 11:38 PM, Jeff Law wrote:

On 02/06/2018 02:38 AM, Aldy Hernandez wrote:

The -Walloca pass can receive a malformed alloca, courtesy of someone
providing a faulty prototype.  This was causing an ICE because we
assumed alloca calls had at least one argument, which the testcase does
not:

+void *alloca ();
+__typeof__(alloca ()) a () { return alloca (); }

I don't believe it should be the responsibility of the
-Walloca-larger-than=* pass to warn against such things, so I propose we
just ignore this.

I also think we should handle this testcase, regardless of the target
having an alloca builtin, since the testcase includes its own
prototype.  Thus, the missing "{dg-require-effect-target alloca}".

Wouldn't it make more sense to put the argument check into
gimple_alloca_call_p that way all callers are insulated?

After all if the alloca doesn't have an argument it probably doesn't
have the semantics of alloca that we expect -- like returning nonzero.

And if we look at vr-values.c:gimple_stmt_nonzero_p we see that it calls
gimple_alloca_call_p and if it returns true, then we assume the call
always returns a nonzero value.   Fixing gimple_alloca_call_p would take
care of both problems at once.

OK with that change.

Attached patch is final version.  I retested on x86-64 Linux.

Committing to trunk and closing PR.

Thanks.

gcc/

	PR tree-optimization/84224
	* gimple-ssa-warn-alloca.c (pass_walloca::execute): Remove assert.
	* calls.c (gimple_alloca_call_p): Only return TRUE when we have
	non-zero arguments.

diff --git a/gcc/calls.c b/gcc/calls.c
index 54fea158631..19c95b8455b 100644
--- a/gcc/calls.c
+++ b/gcc/calls.c
@@ -730,7 +730,7 @@ gimple_alloca_call_p (const gimple *stmt)
     switch (DECL_FUNCTION_CODE (fndecl))
       {
       CASE_BUILT_IN_ALLOCA:
-        return true;
+	return gimple_call_num_args (stmt) > 0;
       default:
 	break;
       }
diff --git a/gcc/gimple-ssa-warn-alloca.c b/gcc/gimple-ssa-warn-alloca.c
index 941810a997e..327c806ae11 100644
--- a/gcc/gimple-ssa-warn-alloca.c
+++ b/gcc/gimple-ssa-warn-alloca.c
@@ -445,7 +445,6 @@ pass_walloca::execute (function *fun)
 
 	  if (!gimple_alloca_call_p (stmt))
 	    continue;
-	  gcc_assert (gimple_call_num_args (stmt) >= 1);
 
 	  const bool is_vla
 	    = gimple_call_alloca_for_var_p (as_a <gcall *> (stmt));
diff --git a/gcc/testsuite/gcc.dg/Walloca-16.c b/gcc/testsuite/gcc.dg/Walloca-16.c
new file mode 100644
index 00000000000..3ee96a9570a
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/Walloca-16.c
@@ -0,0 +1,6 @@
+/* PR tree-optimization/84224 */
+/* { dg-do compile } */
+/* { dg-options "-O0 -Walloca" } */
+
+void *alloca ();
+__typeof__(alloca ()) a () { return alloca (); }