From: Richard Biener <rguenther@suse.de>
Date: Thu, 8 Sep 2022 12:24:32 +0000 (+0200)
Subject: Fix OMP CAS expansion with separate condition
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=6fb06f2628352da3ecf6649a66a1444b598caf59;p=thirdparty%2Fgcc.git

Fix OMP CAS expansion with separate condition

When forcing the condition to be split out from COND_EXPRs I see
a runtime failure of libgomp.fortran/atomic-19.f90 which can be
reduced to

  !$omp atomic update, compare, capture
  if (x == 69_2 - r) x = 6_8
    v = x

being miscompiled, the difference being

-  _13 = .ATOMIC_COMPARE_EXCHANGE (_9, _10, _11, 4, 0, 0);
-  _14 = IMAGPART_EXPR <_13>;
-  _15 = REALPART_EXPR <_13>;
-  _16 = _14 != 0 ? _11 : _15;
-  _2 = (integer(kind=4)) _16;
-  v_17 = _2;
+  _14 = .ATOMIC_COMPARE_EXCHANGE (_10, _11, _12, 4, 0, 0);
+  _15 = IMAGPART_EXPR <_14>;
+  _16 = REALPART_EXPR <_14>;
+  _2 = (logical(kind=1)) _15;
+  _3 = (integer(kind=4)) _16;
+  v_17 = _3;

where one can see a missing COND_EXPR.  It seems to be a latent
issue to me given the code can be exercised, it just maybe misses
a 'need_new' testcase combined with 'cond_stmt'.  Appearantly
the if (cond_stmt) code is just to avoid creating a temporary
(and possibly to preserve the condition compute if used elsewhere
since the original stmt is going to be deleted).  The following
makes the failure go away for me in my patched tree and it
also survives libgomp and gomp testing in an unpatched tree.

2022-05-13  Richard Biener  <rguenther@suse.de>

	* omp-expand.cc (expand_omp_atomic_cas): Do not short-cut
	computation of the new value.

(cherry picked from commit 060173dd73fcaf0767215f9d989ad064e2d5fe2a)
---

diff --git a/gcc/ChangeLog.omp b/gcc/ChangeLog.omp
index f7a0a77780ef..ae066d893ccf 100644
--- a/gcc/ChangeLog.omp
+++ b/gcc/ChangeLog.omp
@@ -1,3 +1,11 @@
+2022-09-08  Tobias Burnus  <tobias@codesourcery.com>
+
+	Backport from mainline:
+	2022-05-13  Richard Biener  <rguenther@suse.de>
+
+	* omp-expand.cc (expand_omp_atomic_cas): Do not short-cut
+	computation of the new value.
+
 2022-09-08  Tobias Burnus  <tobias@codesourcery.com>
 
 	Backport from mainline:
diff --git a/gcc/omp-expand.cc b/gcc/omp-expand.cc
index fa7d3649c83c..f2fd0ee9e90c 100644
--- a/gcc/omp-expand.cc
+++ b/gcc/omp-expand.cc
@@ -9283,16 +9283,17 @@ expand_omp_atomic_cas (basic_block load_bb, tree addr,
 
       if (cond_stmt)
 	{
-	  g = gimple_build_assign (gimple_assign_lhs (cond_stmt),
-				   NOP_EXPR, im);
+	  g = gimple_build_assign (cond, NOP_EXPR, im);
 	  gimple_set_location (g, loc);
 	  gsi_insert_before (&gsi, g, GSI_SAME_STMT);
 	}
-      else if (need_new)
+
+      if (need_new)
 	{
 	  g = gimple_build_assign (create_tmp_reg (itype), COND_EXPR,
-				   build2 (NE_EXPR, boolean_type_node,
-					   im, build_zero_cst (itype)),
+				   cond_stmt
+				   ? cond : build2 (NE_EXPR, boolean_type_node,
+						    im, build_zero_cst (itype)),
 				   d, re);
 	  gimple_set_location (g, loc);
 	  gsi_insert_before (&gsi, g, GSI_SAME_STMT);