[gcc/devel/omp/gcc-12] Fix OMP CAS expansion with separate condition

[Tobias Burnus]

https://gcc.gnu.org/g:6fb06f2628352da3ecf6649a66a1444b598caf59

commit 6fb06f2628352da3ecf6649a66a1444b598caf59
Author: Richard Biener <rguenther@suse.de>
Date:   Thu Sep 8 14:24:32 2022 +0200

    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:
---
 gcc/ChangeLog.omp |  8 ++++++++
 gcc/omp-expand.cc | 11 ++++++-----
 2 files changed, 14 insertions(+), 5 deletions(-)

diff --git a/gcc/ChangeLog.omp b/gcc/ChangeLog.omp
index f7a0a77780e..ae066d893cc 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 fa7d3649c83..f2fd0ee9e90 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);