From: Richard Biener <rguenther@suse.de>
Date: Fri, 4 Aug 2023 09:24:49 +0000 (+0200)
Subject: tree-optimization/110838 - less aggressively fold out-of-bound shifts
X-Git-Tag: basepoints/gcc-15~7154
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=04aa0edcace22a7815cfc57575f1f7b1f166ac10;p=thirdparty%2Fgcc.git

tree-optimization/110838 - less aggressively fold out-of-bound shifts

The following adjusts the shift simplification patterns to avoid
touching out-of-bound shift value arithmetic right shifts of
possibly negative values.  While simplifying those to zero isn't
wrong it's violating the principle of least surprise.

	PR tree-optimization/110838
	* match.pd (([rl]shift @0 out-of-bounds) -> zero): Restrict
	the arithmetic right-shift case to non-negative operands.
---

diff --git a/gcc/match.pd b/gcc/match.pd
index aa49574b22f8..2278029d608b 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -1081,6 +1081,10 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
  (simplify
   (shift @0 uniform_integer_cst_p@1)
   (if ((GIMPLE || !sanitize_flags_p (SANITIZE_SHIFT_EXPONENT))
+       /* Leave arithmetic right shifts of possibly negative values alone.  */
+       && (TYPE_UNSIGNED (type)
+           || shift == LSHIFT_EXPR
+	   || tree_expr_nonnegative_p (@0))
        /* Use a signed compare to leave negative shift counts alone.  */
        && wi::ges_p (wi::to_wide (uniform_integer_cst_p (@1)),
 		     element_precision (type)))