On 15 March 2018 at 11:19, Kyrill Tkachov <kyrylo.tkac...@foss.arm.com> wrote: > 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. >
Hi Tamar, Strangely I have noticed regressions on armeb, I have updated bugzilla accordingly. Thanks, Christophe > Thanks, > Kyrill > > >> Thanks, >> Tamar >> >> gcc/ >> 2018-03-05 Tamar Christina <tamar.christ...@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.christ...@arm.com> >> >> PR target/84711 >> * gcc.target/arm/big-endian-subreg.c: New. >> >> -- > >
