[gcc r12-222] arm: fix UB due to missing mode check [PR100311]

[Richard Earnshaw <rearnsha@gcc.gnu.org>]

https://gcc.gnu.org/g:59f5d16f2c5db4d9592c8ce6453afe81334bb012

commit r12-222-g59f5d16f2c5db4d9592c8ce6453afe81334bb012
Author: Richard Earnshaw <rearnsha@arm.com>
Date:   Wed Apr 28 17:56:38 2021 +0100

    arm: fix UB due to missing mode check [PR100311]
    
    Some places in the compiler iterate over all the fixed registers to
    check if that register can be used in a particular mode.  The idiom is
    to iterate over the register and then for that register, if it
    supports the current mode to check all that register and any
    additional registers needed (HARD_REGNO_NREGS).  If these two checks
    are not fully aligned then it is possible to generate a buffer overrun
    when testing data objects that are sized by the number of hard regs in
    the machine.
    
    The VPR register is a case where these checks were not consistent and
    because this is the last HARD register the result was that we ended up
    overflowing the fixed_regs array.
    
    gcc:
            PR target/100311
            * config/arm/arm.c (arm_hard_regno_mode_ok): Only allow VPR to be
            used in HImode.

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

diff --git a/gcc/config/arm/arm.c b/gcc/config/arm/arm.c
index 6641e3fb4dc..0371d9818fd 100644
--- a/gcc/config/arm/arm.c
+++ b/gcc/config/arm/arm.c
@@ -25269,7 +25269,7 @@ arm_hard_regno_mode_ok (unsigned int regno, machine_mode mode)
     return false;
 
   if (IS_VPR_REGNUM (regno))
-    return true;
+    return mode == HImode;
 
   if (TARGET_THUMB1)
     /* For the Thumb we only allow values bigger than SImode in