[PATCH] have chkp skip flexible member arrays (PR #79986)

[Martin Sebor <msebor@gmail.com>]

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Attached is a minimal patch to avoid an ICE in CHKP upon
encountering one form of an initializer for a flexible array
member, specifically the empty string:

   int f ()
   {
     struct B { int n; char a[]; };

     return ((struct B){ 1, "" }).a[0];
   }

Although GCC accepts (and doesn't ICE on) non-empty initializers
for flexible array members, such as

     (struct B){ 1, "123" }

it generates wrong code for them.  This could either be fixed by
emitting correct code, or it could be handled by rejecting all
initializers for non-static objects with such members.  Both
approaches seem risky to me at this stage and so I think it's
safest to hold off on implementing either until after the release.

Martin

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PR c++/79986 - [CHKP] ICE in fold_convert_loc with a flexible array

gcc/ChangeLog:

	PR c++/79986
	* tree-chkp.c (chkp_process_stmt): Avoid assuming size is non-null.

gcc/testsuite/ChangeLog:

	PR c++/79986
	* g++.dg/pr79986.C: New test.

diff --git a/gcc/testsuite/g++.dg/pr79986.C b/gcc/testsuite/g++.dg/pr79986.C
new file mode 100644
index 0000000..d179cf6
--- /dev/null
+++ b/gcc/testsuite/g++.dg/pr79986.C
@@ -0,0 +1,10 @@
+/* PR c++/79986 - [CHKP] ICE in fold_convert_loc with a flexible array
+   { dg-do compile { target { { i?86-*-* x86_64-*-* } && { ! x32 } } } }
+   { dg-options "-fcheck-pointer-bounds -mmpx" } */
+
+int f (int i)
+{
+  struct A { int n; char a[]; };
+
+  return ((struct A){ 1, "" }).a[i];   // { dg-error "invalid use of array" }
+}
diff --git a/gcc/tree-chkp.c b/gcc/tree-chkp.c
index b1ff218..780d18f 100644
--- a/gcc/tree-chkp.c
+++ b/gcc/tree-chkp.c
@@ -4092,6 +4092,10 @@ chkp_process_stmt (gimple_stmt_iterator *iter, tree node,
      expression to compute it.  */
   if (!addr_last)
     {
+      /* C++ flexible array members have a null size.  */
+      if (!size)
+	return;
+
       addr_last = fold_build_pointer_plus_loc (loc, addr_first, size);
       addr_last = fold_build_pointer_plus_hwi_loc (loc, addr_last, -1);
     }