[PATCH] tree-optimization/107561 - reduce -Wstringop-overflow false positives

[Richard Biener <rguenther@suse.de>]

The following tells pointer-query to prefer a zero size when we
are querying for the size range for a write into an object we've
determined is of zero size.  That avoids diagnostics about really
varying size arguments that just get a meaningful range for example
because they are multiplied by an element size.

I've adjusted only one call to get_size_range since that's what
I have a testcase for.  I think this is the most sensible
"workaround" for some of the false positives we see.

Bootstrapped on x86_64-unknown-linux-gnu, testing in progress.

OK if it succeeds?

Thanks,
Richard.

	PR tree-optimization/107561
	* gimple-ssa-warn-access.cc (get_size_range): Add flags
	argument and pass it on.
	(check_access): When querying for the size range pass
	SR_ALLOW_ZERO when the known destination size is zero.

	* g++.dg/pr71488.C: Remove XFAILed bogus diagnostic again.
	* g++.dg/warn/Warray-bounds-16.C: Likewise.
---
 gcc/gimple-ssa-warn-access.cc                | 19 +++++++++++++------
 gcc/testsuite/g++.dg/pr71488.C               |  1 -
 gcc/testsuite/g++.dg/warn/Warray-bounds-16.C |  1 -
 3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/gcc/gimple-ssa-warn-access.cc b/gcc/gimple-ssa-warn-access.cc
index 88d44690ade..b3de4b77924 100644
--- a/gcc/gimple-ssa-warn-access.cc
+++ b/gcc/gimple-ssa-warn-access.cc
@@ -1198,10 +1198,10 @@ warn_for_access (location_t loc, tree func, tree expr, int opt,
 
 static void
 get_size_range (range_query *query, tree bound, gimple *stmt, tree range[2],
-		const offset_int bndrng[2])
+		int flags, const offset_int bndrng[2])
 {
   if (bound)
-    get_size_range (query, bound, stmt, range);
+    get_size_range (query, bound, stmt, range, flags);
 
   if (!bndrng || (bndrng[0] == 0 && bndrng[1] == HOST_WIDE_INT_M1U))
     return;
@@ -1347,7 +1347,12 @@ check_access (GimpleOrTree exp, tree dstwrite,
   /* Set RANGE to that of DSTWRITE if non-null, bounded by PAD->DST_BNDRNG
      if valid.  */
   gimple *stmt = pad ? pad->stmt : nullptr;
-  get_size_range (rvals, dstwrite, stmt, range, pad ? pad->dst_bndrng : NULL);
+  get_size_range (rvals, dstwrite, stmt, range,
+		  /* If the destination has known zero size prefer a zero
+		     size range to avoid false positives if that's a
+		     possibility.  */
+		  integer_zerop (dstsize) ? SR_ALLOW_ZERO : 0,
+		  pad ? pad->dst_bndrng : NULL);
 
   tree func = get_callee_fndecl (exp);
   /* Read vs write access by built-ins can be determined from the const
@@ -1442,7 +1447,8 @@ check_access (GimpleOrTree exp, tree dstwrite,
     {
       /* Set RANGE to that of MAXREAD, bounded by PAD->SRC_BNDRNG if
 	 PAD is nonnull and BNDRNG is valid.  */
-      get_size_range (rvals, maxread, stmt, range, pad ? pad->src_bndrng : NULL);
+      get_size_range (rvals, maxread, stmt, range, 0,
+		      pad ? pad->src_bndrng : NULL);
 
       location_t loc = get_location (exp);
       tree size = dstsize;
@@ -1489,7 +1495,8 @@ check_access (GimpleOrTree exp, tree dstwrite,
     {
       /* Set RANGE to that of MAXREAD, bounded by PAD->SRC_BNDRNG if
 	 PAD is nonnull and BNDRNG is valid.  */
-      get_size_range (rvals, maxread, stmt, range, pad ? pad->src_bndrng : NULL);
+      get_size_range (rvals, maxread, stmt, range, 0,
+		      pad ? pad->src_bndrng : NULL);
       /* Set OVERREAD for reads starting just past the end of an object.  */
       overread = pad->src.sizrng[1] - pad->src.offrng[0] < pad->src_bndrng[0];
       range[0] = wide_int_to_tree (sizetype, pad->src_bndrng[0]);
@@ -2679,7 +2686,7 @@ pass_waccess::check_strncmp (gcall *stmt)
   /* Determine the range of the bound first and bail if it fails; it's
      cheaper than computing the size of the objects.  */
   tree bndrng[2] = { NULL_TREE, NULL_TREE };
-  get_size_range (m_ptr_qry.rvals, bound, stmt, bndrng, adata1.src_bndrng);
+  get_size_range (m_ptr_qry.rvals, bound, stmt, bndrng, 0, adata1.src_bndrng);
   if (!bndrng[0] || integer_zerop (bndrng[0]))
     return;
 
diff --git a/gcc/testsuite/g++.dg/pr71488.C b/gcc/testsuite/g++.dg/pr71488.C
index a89e6465a94..ffe30f1afeb 100644
--- a/gcc/testsuite/g++.dg/pr71488.C
+++ b/gcc/testsuite/g++.dg/pr71488.C
@@ -3,7 +3,6 @@
 // { dg-options "-O3 -std=c++11" }
 // { dg-additional-options "-msse4" { target sse4_runtime } }
 // { dg-require-effective-target c++11 }
-// { dg-bogus "size 0 overflows" "pr107561" { xfail *-*-* } 0 }
 
 #include <valarray>
 
diff --git a/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C b/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
index 49d3437fa1b..89cbadb91c7 100644
--- a/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
+++ b/gcc/testsuite/g++.dg/warn/Warray-bounds-16.C
@@ -20,7 +20,6 @@ struct S
 
     for (int i = 0; i < m; i++)
       new (p + i) int (); /* { dg-bogus "bounds" "pr102690" { xfail *-*-* } } */
-    // { dg-bogus "size 0 overflows" "pr107561" { xfail ilp32 } .-1 }
   }
 };
 
-- 
2.35.3