RE: [PATCH][GCC][ARM] Fix can_change_mode_class for big-endian

[Tamar Christina <Tamar dot Christina at arm dot com>]

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> -----Original Message-----
> From: gcc-patches-owner@gcc.gnu.org [mailto:gcc-patches-
> owner@gcc.gnu.org] On Behalf Of Tamar Christina
> Sent: Monday, March 5, 2018 16:52
> To: gcc-patches@gcc.gnu.org
> Cc: nd <nd@arm.com>; Ramana Radhakrishnan
> <Ramana.Radhakrishnan@arm.com>; Richard Earnshaw
> <Richard.Earnshaw@arm.com>; nickc@redhat.com; Kyrylo Tkachov
> <Kyrylo.Tkachov@arm.com>
> Subject: [PATCH][GCC][ARM] Fix can_change_mode_class for big-endian
> 
> 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?
> 
> 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.
> 
> --