[gcc(refs/vendors/ARM/heads/morello)] dwarf2out: Avoid creating rtx with mode MAX_MACHINE_MODE

[gcc(refs/vendors/ARM/heads/morello)] dwarf2out: Avoid creating rtx with mode MAX_MACHINE_MODE

[Matthew Malcomson]

https://gcc.gnu.org/g:a16e2e017098bc895d12bf08402bdfab65a7d11d

commit a16e2e017098bc895d12bf08402bdfab65a7d11d
Author: Alex Coplan <alex.coplan@arm.com>
Date:   Thu Nov 18 13:53:16 2021 +0000

    dwarf2out: Avoid creating rtx with mode MAX_MACHINE_MODE
    
    This patch fixes an off-by-one out-of-bounds access of a global variable
    found by bootstrapping --with-build-config=bootstrap-asan. With this
    patch applied, the compiler now succesfully bootstraps in this
    configuration.
    
    The issue here is that the code in dwarf2out.c abuses the mode of rtxes
    (specifically EXPR_LISTs) to store some extra information. In
    particular, with the testcase below, this code can end up creating an
    EXPR_LIST rtx with the mode set to MAX_MACHINE_MODE. This is problematic
    because MAX_MACHINE_MODE itself is not a valid mode: all of the arrays
    (mapping modes to some other entity) in the generated insn-modes.c only
    contain MAX_MACHINE_MODE entries, so MAX_MACHINE_MODE is not a valid
    index to these arrays (it is one past the end of the array).
    
    The reason we only see this with the Morello compiler is due to the
    check_code_and_mode check invoked on the creation of any rtx. This
    function then ends up using CAPABILITY_MODE_P on MAX_MACHINE_MODE, which
    expands to a use of the GET_MODE_CLASS macro, which accesses one off the
    end of the mode_class array (defined in the generated insn-modes.c).
    
    Looking at dwarf2out.c:decl_piece_node, it seems that abusing the mode
    in this way is simply a trick to avoid allocating an extra rtx to store
    the bitsize argument, so in this patch we just force the code to
    allocate the extra rtx (which can represent any integer) for the case of
    bitsize == MAX_MACHINE_MODE.
    
    Testcase that shows the problem (compile with -O -g):
    
    typedef struct {
      __int128 ll;
    } DWunion;
    void __divti3() { DWunion vv = {.0}; }
    
    gcc/ChangeLog:
    
            * dwarf2out.c (decl_piece_node): Avoid creating an EXPR_LIST
            with a mode of MAX_MACHINE_MODE.

Diff:
---
 gcc/dwarf2out.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/gcc/dwarf2out.c b/gcc/dwarf2out.c
index 9c0327d7209..2e3ff1134db 100644
--- a/gcc/dwarf2out.c
+++ b/gcc/dwarf2out.c
@@ -6141,7 +6141,7 @@ decl_piece_varloc_ptr (rtx piece)
 static rtx_expr_list *
 decl_piece_node (rtx loc_note, HOST_WIDE_INT bitsize, rtx next)
 {
-  if (bitsize > 0 && bitsize <= (int) MAX_MACHINE_MODE)
+  if (bitsize > 0 && bitsize < (int) MAX_MACHINE_MODE)
     return alloc_EXPR_LIST (bitsize, loc_note, next);
   else
     return alloc_EXPR_LIST (0, gen_rtx_CONCAT (VOIDmode,