diff --git a/gcc.spec b/gcc.spec
index 1fcaa56..f2ac97e 100644
--- a/gcc.spec
+++ b/gcc.spec
@@ -266,6 +266,7 @@ Patch11: gcc11-rh1574936.patch
 Patch12: gcc11-d-shared-libphobos.patch
 Patch13: gcc11-pr99341-revert.patch
 Patch14: gcc11-pr100379.patch
+Patch15: gcc11-pr100566.patch
 
 Patch100: gcc11-fortran-fdec-duplicates.patch
 Patch101: gcc11-fortran-flogical-as-integer.patch
@@ -790,6 +791,7 @@ to NVidia PTX capable devices if available.
 %patch12 -p0 -b .d-shared-libphobos~
 %patch13 -p0 -b .pr99341-revert~
 %patch14 -p0 -b .pr100379~
+%patch15 -p0 -b .pr100566~
 
 %if 0%{?rhel} >= 9
 %patch100 -p1 -b .fortran-fdec-duplicates~
@@ -3145,6 +3147,7 @@ end
 	tree-optimization/100253, tree-optimization/100278,
 	tree-optimization/100329, tree-optimization/100414
 - fix build with removed linux/cyclades.h header (PR sanitizer/100379)
+- fix up mausezahn miscompilation (#1958887, PR tree-optimization/100566)
 
 * Wed Apr 28 2021 Jakub Jelinek <jakub@redhat.com> 11.1.1-1
 - update from releases/gcc-11-branch
diff --git a/gcc11-pr100566.patch b/gcc11-pr100566.patch
new file mode 100644
index 0000000..6134be2
--- /dev/null
+++ b/gcc11-pr100566.patch
@@ -0,0 +1,69 @@
+2021-05-12  Richard Biener  <rguenther@suse.de>
+
+	PR tree-optimization/100566
+	* tree-ssa-sccvn.c (dominated_by_p_w_unex): Properly handle
+	allow_back for all edge queries.
+
+	* gcc.dg/torture/pr100566.c: New testcase.
+
+--- gcc/tree-ssa-sccvn.c
++++ gcc/tree-ssa-sccvn.c
+@@ -4529,7 +4529,8 @@ dominated_by_p_w_unex (basic_block bb1, basic_block bb2, bool allow_back)
+   /* Iterate to the single executable bb2 successor.  */
+   edge succe = NULL;
+   FOR_EACH_EDGE (e, ei, bb2->succs)
+-    if (e->flags & EDGE_EXECUTABLE)
++    if ((e->flags & EDGE_EXECUTABLE)
++	|| (!allow_back && (e->flags & EDGE_DFS_BACK)))
+       {
+ 	if (succe)
+ 	  {
+@@ -4547,7 +4548,8 @@ dominated_by_p_w_unex (basic_block bb1, basic_block bb2, bool allow_back)
+ 	{
+ 	  FOR_EACH_EDGE (e, ei, succe->dest->preds)
+ 	    if (e != succe
+-		&& (e->flags & EDGE_EXECUTABLE))
++		&& ((e->flags & EDGE_EXECUTABLE)
++		    || (!allow_back && (e->flags & EDGE_DFS_BACK))))
+ 	      {
+ 		succe = NULL;
+ 		break;
+--- gcc/testsuite/gcc.dg/torture/pr100566.c
++++ gcc/testsuite/gcc.dg/torture/pr100566.c
+@@ -0,0 +1,36 @@
++/* { dg-do run } */
++
++volatile int s, c;
++
++__attribute__((noipa)) void
++foo (void)
++{
++  if (c++ > 1)
++    __builtin_abort ();
++}
++
++__attribute__((noipa)) int
++bar (void)
++{
++  int i = 0, j = s;
++  if (j == 0)
++    goto lab;
++  for (i = 0; i < j; i++)
++    {
++    lab:
++      foo ();
++      if (!j)
++        goto lab;
++    }
++  return 0;
++}
++
++int
++main ()
++{
++  s = 1;
++  bar ();
++  if (c != 1)
++    __builtin_abort ();
++  return 0;
++}