[PATCH][GCC11] tree-optimization/104931 - mitigate niter analysis issue

[PATCH][GCC11] tree-optimization/104931 - mitigate niter analysis issue

[Richard Biener]

The following backports a pointer associating pattern from trunk
that mitigates an issue with number_of_iterations_lt_to_ne in
which context we fail to fold a comparison but succeed in folding
a related subtraction.  In the failure mode this results in
a loop wrongly assumed looping with a bogus number of iterations,
resulting in crashing of the premake application on start.

With the backported simplification we are able to fold the
comparison and correctly compute the loop as not iterating.

I have failed to create a standalone testcase.  I belive part
of the issue is still latent but I have failed to nail down
the issue exactly.  Still I believe the backporting of the
mitigation patch is the most sensible and safe thing at this
point.

Bootstrapped and tested on x86_64-unknown-linux-gnu on the branch,
OK?

Thanks,
Richard.

2022-03-16  Richard Biener  <rguenther@suse.de>

	PR tree-optimization/104931
	* match.pd ((ptr) (x p+ y) p+ z -> (ptr) (x p+ (y + z))): New GENERIC
	simplification.
---
 gcc/match.pd | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/gcc/match.pd b/gcc/match.pd
index 05a08d0f96a..a005dcd42bd 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -1997,6 +1997,11 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
 (simplify
   (pointer_plus (pointer_plus:s @0 @1) @3)
   (pointer_plus @0 (plus @1 @3)))
+#if GENERIC
+(simplify
+  (pointer_plus (convert:s (pointer_plus:s @0 @1)) @3)
+  (convert:type (pointer_plus @0 (plus @1 @3))))
+#endif
 
 /* Pattern match
      tem1 = (long) ptr1;
-- 
2.34.1