[gcc r13-2033] Improve comment for tree_niter_desc.{control, bound, cmp}

[gcc r13-2033] Improve comment for tree_niter_desc.{control, bound, cmp}

[Andrew Carlotti]

https://gcc.gnu.org/g:88ee126ee0401e293888e7d1b9219ad35eb130b5

commit r13-2033-g88ee126ee0401e293888e7d1b9219ad35eb130b5
Author: Andrew Carlotti <andrew.carlotti@arm.com>
Date:   Fri Aug 12 14:36:22 2022 +0100

    Improve comment for tree_niter_desc.{control,bound,cmp}
    
    Fix typos and explain ERROR_MARK usage.
    
    gcc/ChangeLog:
    
            * tree-ssa-loop.h: Improve comment

Diff:
---
 gcc/tree-ssa-loop.h | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/gcc/tree-ssa-loop.h b/gcc/tree-ssa-loop.h
index 415f461c37e..6c70f795d17 100644
--- a/gcc/tree-ssa-loop.h
+++ b/gcc/tree-ssa-loop.h
@@ -54,11 +54,11 @@ public:
   widest_int max;	/* The upper bound on the number of iterations of
 			   the loop.  */
 
-  /* The simplified shape of the exit condition.  The loop exits if
-     CONTROL CMP BOUND is false, where CMP is one of NE_EXPR,
-     LT_EXPR, or GT_EXPR, and step of CONTROL is positive if CMP is
-     LE_EXPR and negative if CMP is GE_EXPR.  This information is used
-     by loop unrolling.  */
+  /* The simplified shape of the exit condition.  This information is used by
+     loop unrolling.  If CMP is ERROR_MARK, then the loop cannot be unrolled.
+     Otherwise, the loop exits if CONTROL CMP BOUND is false, where CMP is one
+     of NE_EXPR, LT_EXPR, or GT_EXPR, and CONTROL.STEP is positive if CMP is
+     LT_EXPR and negative if CMP is GT_EXPR.  */
   affine_iv control;
   tree bound;
   enum tree_code cmp;