Re: [PATCH] tree-optimization/91838 - fix FAIL of g++.dg/opt/pr91838.C

[Richard Biener <rguenther@suse.de>]

On Thu, 27 Jul 2023, Jakub Jelinek wrote:

> On Thu, Jul 27, 2023 at 12:00:56PM +0000, Richard Biener wrote:
> > The following fixes the lack of simplification of a vector shift
> > by an out-of-bounds shift value.  For scalars this is done both
> > by CCP and VRP but vectors are not handled there.  This results
> > in PR91838 differences in outcome dependent on whether a vector
> > shift ISA is available and thus vector lowering does or does not
> > expose scalar shifts here.
> > 
> > The following adds a match.pd pattern to catch uniform out-of-bound
> > shifts, simplifying them to zero when not sanitizing shift amounts.
> > 
> > Bootstrapped and tested on x86_64-unknown-linux-gnu.
> > 
> > OK?
> > 
> > Thanks,
> > Richard.
> > 
> > 	PR tree-optimization/91838
> > 	* match.pd (([rl]shift @0 out-of-bounds) -> zero): New pattern.
> 
> The !(flag_sanitize & SANITIZE_SHIFT_EXPONENT)
> should be !sanitize_flags_p (SANITIZE_SHIFT_EXPONENT)
> or maybe even
> GIMPLE || !sanitize_flags_p (SANITIZE_SHIFT_EXPONENT)
> because the shift ubsan instrumentation is done on GENERIC, so it can be
> optimized on GIMPLE even with ubsan.
> 
> Otherwise LGTM.

So like the following, will push after re-testing succeeded.

Thanks,
Richard.

From c6d348acdc2143fc4c2849e33075a3975fe29b26 Mon Sep 17 00:00:00 2001
From: Richard Biener <rguenther@suse.de>
Date: Thu, 27 Jul 2023 13:08:32 +0200
Subject: [PATCH] tree-optimization/91838 - fix FAIL of g++.dg/opt/pr91838.C
To: gcc-patches@gcc.gnu.org

The following fixes the lack of simplification of a vector shift
by an out-of-bounds shift value.  For scalars this is done both
by CCP and VRP but vectors are not handled there.  This results
in PR91838 differences in outcome dependent on whether a vector
shift ISA is available and thus vector lowering does or does not
expose scalar shifts here.

The following adds a match.pd pattern to catch uniform out-of-bound
shifts, simplifying them to zero when not sanitizing shift amounts.

	PR tree-optimization/91838
	* gimple-match-head.cc: Include attribs.h and asan.h.
	* generic-match-head.cc: Likewise.
	* match.pd (([rl]shift @0 out-of-bounds) -> zero): New pattern.
---
 gcc/generic-match-head.cc |  2 ++
 gcc/gimple-match-head.cc  |  2 ++
 gcc/match.pd              | 10 ++++++++++
 3 files changed, 14 insertions(+)

diff --git a/gcc/generic-match-head.cc b/gcc/generic-match-head.cc
index b4b5bc88f4b..a71c0727b0b 100644
--- a/gcc/generic-match-head.cc
+++ b/gcc/generic-match-head.cc
@@ -41,6 +41,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "tree-eh.h"
 #include "langhooks.h"
 #include "tree-pass.h"
+#include "attribs.h"
+#include "asan.h"
 
 /* Routine to determine if the types T1 and T2 are effectively
    the same for GENERIC.  If T1 or T2 is not a type, the test
diff --git a/gcc/gimple-match-head.cc b/gcc/gimple-match-head.cc
index d795066e53e..5d6d26d009b 100644
--- a/gcc/gimple-match-head.cc
+++ b/gcc/gimple-match-head.cc
@@ -47,6 +47,8 @@ along with GCC; see the file COPYING3.  If not see
 #include "tm.h"
 #include "gimple-range.h"
 #include "langhooks.h"
+#include "attribs.h"
+#include "asan.h"
 
 tree do_valueize (tree, tree (*)(tree), bool &);
 tree do_valueize (tree (*)(tree), tree);
diff --git a/gcc/match.pd b/gcc/match.pd
index a443dc48634..fcb1a735507 100644
--- a/gcc/match.pd
+++ b/gcc/match.pd
@@ -1059,6 +1059,16 @@ DEFINE_INT_AND_FLOAT_ROUND_FN (RINT)
        && tree_nop_conversion_p (type, TREE_TYPE (@1)))
    (lshift @0 @2)))
 
+/* Shifts by precision or greater result in zero.  */
+(for shift (lshift rshift)
+ (simplify
+  (shift @0 uniform_integer_cst_p@1)
+  (if ((GIMPLE || !sanitize_flags_p (SANITIZE_SHIFT_EXPONENT))
+       /* Use a signed compare to leave negative shift counts alone.  */
+       && wi::ges_p (wi::to_wide (uniform_integer_cst_p (@1)),
+		     element_precision (type)))
+   { build_zero_cst (type); })))
+
 /* Shifts by constants distribute over several binary operations,
    hence (X << C) + (Y << C) can be simplified to (X + Y) << C.  */
 (for op (plus minus)
-- 
2.35.3

k