[Bug tree-optimization/94802] Failure to recognize identities with __builtin_clz

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94802

--- Comment #4 from Jakub Jelinek <jakub at gcc dot gnu.org> ---
Not fixed completely yet, (a - b) < 0 is not optimized into a < b yet.
The following patch does that, but it regresses 
--- gcc/match.pd.jj     2020-12-30 17:04:27.340815135 +0100
+++ gcc/match.pd        2020-12-30 17:41:44.393443188 +0100
@@ -3966,6 +3966,16 @@ (define_operator_list COND_TERNARY
    (cmp (sub@2 @0 @1) integer_zerop)
    (if (single_use (@2))
     (cmp @0 @1)))))
+#if GIMPLE
+(for cmp (lt le gt ge)
+ (for sub (minus pointer_diff)
+  (simplify
+   (cmp (sub@2 @0 @1) integer_zerop)
+   (if (TYPE_OVERFLOW_UNDEFINED (TREE_TYPE (@0))
+       && !TYPE_OVERFLOW_SANITIZED (TREE_TYPE (@0))
+       && single_use (@2))
+    (cmp @0 @1)))))
+#endif

 /* Transform comparisons of the form X * C1 CMP 0 to X CMP 0 in the
    signed arithmetic case.  That form is created by the compiler
the following test:
+FAIL: gcc.dg/tree-ssa/pr45397.c scan-tree-dump-not evrp " & 255;"
+FAIL: gcc.dg/tree-ssa/pr45397.c scan-tree-dump-not phiopt2 "if "
+FAIL: gcc.dg/tree-ssa/pr45397.c scan-tree-dump-times phiopt2 "MAX_EXPR" 3
+FAIL: gcc.dg/tree-ssa/pr45397.c scan-tree-dump-times phiopt2 "MIN_EXPR" 3
so some more work will be needed.