On Tue, Jun 30, 2026 at 2:55 PM Konstantinos Eleftheriou <[email protected]> wrote: > > `x` is store_bit_field's `str_rtx`, the V4x4BF register. You can see ASF's > dumps in this comment > in the PR: > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=125988#c6 > > `x` is `(reg:V4x4BF 60 v28)` in this case.
How does copy_to_reg for a (reg:V4x4BF ..) help then? Is that hardreg vs pseudo and "somehow" validate_subreg is happy with one but not the other? > Konstantinos. > > On Tue, Jun 30, 2026 at 3:36 PM Richard Biener <[email protected]> > wrote: >> >> On Tue, Jun 30, 2026 at 2:20 PM Konstantinos Eleftheriou >> <[email protected]> wrote: >> > >> > The call chain for our testcase is the following: >> > gen_lowpart -> gen_lowpart_general -> gen_lowpart_common -> lowpart_subreg >> > -> simplify_gen_subreg -> validate_subreg. >> > >> > `validate_subreg` for OI and V4x4BF modes fails, leading to this fallback >> > in `gen_lowpart_general`: >> > >> > /* Handle SUBREGs and hard REGs that were rejected by >> > simplify_gen_subreg. */ >> > else if (REG_P (x) || GET_CODE (x) == SUBREG) >> > { >> > result = gen_lowpart_common (mode, copy_to_reg (x)); >> > gcc_assert (result != 0); >> > return result; >> > } >> > >> > `copy_to_reg (x)` creates the new reg. >> >> And what's 'x' here? That said, either gen_lowpart () isn't supposed >> to be used in >> lvalue context or there's sth else wrong around store_bit_field >> (not to say, OI and V4x4BF? really?) >> >> Richard. >> >> > >> > >> > >> > >> > On Tue, Jun 30, 2026 at 1:57 PM Richard Biener >> > <[email protected]> wrote: >> >> >> >> On Tue, Jun 30, 2026 at 11:22 AM Konstantinos Eleftheriou >> >> <[email protected]> wrote: >> >> > >> >> > Hi Richard, >> >> > >> >> > We've been sending this as part of the ASF default enablement patchset >> >> > for nearly a year now. >> >> > We are splitting it now as we judged that it requires separate >> >> > attention. >> >> > >> >> > The idea is to overwrite the original to-be-updated register -- >> >> > stripping the subregs to extract it -- >> >> > with the newly generated one. Otherwise, str_rtx is left unchanged. >> >> > The copy is done by `emit_mov_insn`. >> >> > >> >> > Any ideas on how we could handle this better? It definitely looks like >> >> > the right place for >> >> > the fix though. >> >> >> >> I don't think changing the destination was intended when doing >> >> >> >> op0 = gen_lowpart (int_mode_for_mode (GET_MODE (op0), op0); >> >> >> >> but instead a subreg was intended (and it's full-size). I wasn't >> >> aware that gen_lowpart >> >> eventually creates a new reg -- when would it do that? >> >> >> >> > >> >> > Thanks, >> >> > Konstantinos >> >> > >> >> > On Tue, Jun 30, 2026 at 10:22 AM Richard Biener >> >> > <[email protected]> wrote: >> >> >> >> >> >> On Mon, Jun 29, 2026 at 4:13 PM Konstantinos Eleftheriou >> >> >> <[email protected]> wrote: >> >> >> > >> >> >> > The call to `gen_lowpart` in `store_bit_field_1` might copy the >> >> >> > destination >> >> >> > register into a new one, which may lead to wrong code generation, as >> >> >> > the bit >> >> >> > insertions update the new register instead of updating `str_rtx`. >> >> >> > >> >> >> > This patch copies back the new destination register into `str_rtx` >> >> >> > when needed. >> >> >> > >> >> >> > Bootstrapped/regtested on AArch64 and x86-64. >> >> >> > >> >> >> > PR rtl-optimization/125988 >> >> >> > >> >> >> > gcc/ChangeLog: >> >> >> > >> >> >> > * expmed.cc (store_bit_field_1): Copy back the new >> >> >> > destination >> >> >> > register into `str_rtx` when needed. >> >> >> > >> >> >> > gcc/testsuite/ChangeLog: >> >> >> > >> >> >> > * gcc.target/aarch64/pr125988.c: New test. >> >> >> > --- >> >> >> > gcc/expmed.cc | 22 +++++++-- >> >> >> > gcc/testsuite/gcc.target/aarch64/pr125988.c | 51 >> >> >> > +++++++++++++++++++++ >> >> >> > 2 files changed, 70 insertions(+), 3 deletions(-) >> >> >> > create mode 100644 gcc/testsuite/gcc.target/aarch64/pr125988.c >> >> >> > >> >> >> > diff --git a/gcc/expmed.cc b/gcc/expmed.cc >> >> >> > index da1b5b632876..1f4611a6ed89 100644 >> >> >> > --- a/gcc/expmed.cc >> >> >> > +++ b/gcc/expmed.cc >> >> >> > @@ -888,9 +888,25 @@ store_bit_field_1 (rtx str_rtx, poly_uint64 >> >> >> > bitsize, poly_uint64 bitnum, >> >> >> > op0 = gen_lowpart (op0_mode.require (), op0); >> >> >> > } >> >> >> > >> >> >> > - return store_integral_bit_field (op0, op0_mode, ibitsize, ibitnum, >> >> >> > - bitregion_start, bitregion_end, >> >> >> > - fieldmode, value, reverse, >> >> >> > fallback_p); >> >> >> > + if (!store_integral_bit_field (op0, op0_mode, ibitsize, ibitnum, >> >> >> > + bitregion_start, bitregion_end, >> >> >> > + fieldmode, value, reverse, >> >> >> > fallback_p)) >> >> >> > + return false; >> >> >> > + >> >> >> > + rtx op0_reg = op0; >> >> >> > + rtx str_rtx_reg = str_rtx; >> >> >> > + while (SUBREG_P (op0_reg)) >> >> >> > + op0_reg = SUBREG_REG (op0_reg); >> >> >> > + while (SUBREG_P (str_rtx_reg)) >> >> >> > + str_rtx_reg = SUBREG_REG (str_rtx_reg); >> >> >> >> >> >> That looks definitely fishy. >> >> >> >> >> >> You do not quote the part that does the copy, but stripping all subregs >> >> >> and then copying looks wrong. It also looks this was produced by an >> >> >> AI? >> >> >> >> >> >> > + >> >> >> > + /* If a new destination register has been generated, copy the >> >> >> > value back >> >> >> > + into str_rtx. */ >> >> >> > + if (REG_P (op0_reg) && REG_P (str_rtx_reg) >> >> >> > + && REGNO (op0_reg) != REGNO (str_rtx_reg)) >> >> >> > + emit_move_insn (str_rtx_reg, op0_reg); >> >> >> > + >> >> >> > + return true; >> >> >> > } >> >> >> > >> >> >> > /* Subroutine of store_bit_field_1, with the same arguments, except >> >> >> > diff --git a/gcc/testsuite/gcc.target/aarch64/pr125988.c >> >> >> > b/gcc/testsuite/gcc.target/aarch64/pr125988.c >> >> >> > new file mode 100644 >> >> >> > index 000000000000..3ac7be9b7b99 >> >> >> > --- /dev/null >> >> >> > +++ b/gcc/testsuite/gcc.target/aarch64/pr125988.c >> >> >> > @@ -0,0 +1,51 @@ >> >> >> > +/* PR rtl-optimization/125988 */ >> >> >> > +/* { dg-do run } */ >> >> >> > +/* { dg-require-effective-target arm_v8_2a_bf16_neon_ok } */ >> >> >> > +/* { dg-options "-O3 -favoid-store-forwarding" } */ >> >> >> > +/* { dg-add-options arm_v8_2a_bf16_neon } */ >> >> >> > + >> >> >> > +/* Verify that the lane inserted by vld4_lane_bf16 survives >> >> >> > + avoid-store-forwarding's bit-insert rewrite. */ >> >> >> > + >> >> >> > +#include <arm_neon.h> >> >> >> > + >> >> >> > +extern void abort (void); >> >> >> > + >> >> >> > +typedef union >> >> >> > +{ >> >> >> > + bfloat16_t bf; >> >> >> > + unsigned short u; >> >> >> > +} bf16_u; >> >> >> > + >> >> >> > +__attribute__((noipa)) static int >> >> >> > +test (const bf16_u *data, const bf16_u *overwrite) >> >> >> > +{ >> >> >> > + bfloat16x4x4_t v; >> >> >> > + bf16_u t[4]; >> >> >> > + int i, j; >> >> >> > + for (i = 0; i < 4; i++, data += 4) >> >> >> > + v.val[i] = vld1_bf16 (&data->bf); >> >> >> > + v = vld4_lane_bf16 (&overwrite->bf, v, 3); >> >> >> > + while (--i >= 0) >> >> >> > + { >> >> >> > + vst1_bf16 (&t[0].bf, v.val[i]); >> >> >> > + data -= 4; >> >> >> > + for (j = 0; j < 4; j++) >> >> >> > + if (t[j].u != (j == 3 ? overwrite[i].u : data[j].u)) >> >> >> > + return 1; >> >> >> > + } >> >> >> > + return 0; >> >> >> > +} >> >> >> > + >> >> >> > +int >> >> >> > +main (void) >> >> >> > +{ >> >> >> > + bf16_u d[16]; >> >> >> > + for (int i = 0; i < 16; i++) >> >> >> > + d[i].u = 0x1000 + i; >> >> >> > + bf16_u ov[4] = { {.u = 0xABCD}, {.u = 0x1234}, >> >> >> > + {.u = 0xCAFE}, {.u = 0xBEEF} }; >> >> >> > + if (test (d, ov)) >> >> >> > + abort (); >> >> >> > + return 0; >> >> >> > +} >> >> >> > -- >> >> >> > 2.52.0 >> >> >> >
