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Re: [PATCH][GCC][ARM] Fix can_change_mode_class for big-endian
- From: Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>
- To: Tamar Christina <Tamar dot Christina at arm dot com>, "gcc-patches at gcc dot gnu dot org" <gcc-patches at gcc dot gnu dot org>
- Cc: Ramana Radhakrishnan <Ramana dot Radhakrishnan at arm dot com>, Richard Earnshaw <Richard dot Earnshaw at arm dot com>, "nickc at redhat dot com" <nickc at redhat dot com>
- Date: Thu, 15 Mar 2018 10:19:57 +0000
- Subject: Re: [PATCH][GCC][ARM] Fix can_change_mode_class for big-endian
- References: <20180305165139.GA27394@arm.com>
Hi Tamar,
On 05/03/18 16:51, Tamar Christina wrote:
Hi All,
Taking the subreg of a vector mode on big-endian may result in an infinite
recursion and eventually a segfault once we run out of stack space.
As an example, taking a subreg of V4HF to SImode we end up in the following
loop on big-endian:
#861 0x00000000008462e9 in operand_subword_force src/gcc/gcc/emit-rtl.c:1787
#862 0x0000000000882a90 in emit_move_multi_word src/gcc/gcc/expr.c:3621
#863 0x000000000087eea1 in emit_move_insn_1 src/gcc/gcc/expr.c:3698
#864 0x000000000087f350 in emit_move_insn src/gcc/gcc/expr.c:3757
#865 0x000000000085e326 in copy_to_reg src/gcc/gcc/explow.c:603
#866 0x00000000008462e9 in operand_subword_force src/gcc/gcc/emit-rtl.c:1787
The reason is that operand_subword_force will always fail. When the value is in
a register that can't be accessed as a multi word the code tries to create a new
psuedo register and emit the value to it. Eventually you end up in simplify_gen_subreg
which calls validate_subreg.
validate_subreg will however always fail because of the REG_CAN_CHANGE_MODE_P check.
On little endian this check always returns true. On big-endian this check is supposed
to prevent values that have a size larger than word size, due to those being stored in
VFP registers.
However we are only interested in a subreg of the vector mode, so we should be checking
the unit size, not the size of the entire mode. Doing this fixes the problem.
Regtested on armeb-none-eabi and no regressions.
Bootstrapped on arm-none-linux-gnueabihf and no issues.
Ok for trunk? and for backport to GCC 7?
Ok for trunk.
Please wait for a few days before backporting.
Thanks,
Kyrill
Thanks,
Tamar
gcc/
2018-03-05 Tamar Christina <tamar.christina@arm.com>
PR target/84711
* config/arm/arm.c (arm_can_change_mode_class): Use GET_MODE_UNIT_SIZE
instead of GET_MODE_SIZE when comparing Units.
gcc/testsuite/
2018-03-05 Tamar Christina <tamar.christina@arm.com>
PR target/84711
* gcc.target/arm/big-endian-subreg.c: New.
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