Richard Sandiford <richard.sandif...@arm.com> writes:
> Jeff Law <j...@ventanamicro.com> writes:
>> [...]
>> + if (GET_CODE (x) == ZERO_EXTRACT)
>> + {
>> + /* If either the size or the start position is unknown,
>> + then assume we know nothing about what is overwritten.
>> + This is overly conservative, but safe. */
>> + if (!CONST_INT_P (XEXP (x, 1)) || !CONST_INT_P (XEXP (x, 2)))
>> + continue;
>> + mask = (1ULL << INTVAL (XEXP (x, 1))) - 1;
>> + bit = INTVAL (XEXP (x, 2));
>> + if (BITS_BIG_ENDIAN)
>> + bit = (GET_MODE_BITSIZE (GET_MODE (x))
>> + - INTVAL (XEXP (x, 1)) - bit).to_constant ();
>> + x = XEXP (x, 0);
>
> Should mask be shifted by bit here, like it is for the subregs? E.g.:
>
> (set (zero_extract:SI R (const_int 2) (const_int 7)))
>
> would currently give a mask of 0x3 and a bit of 7, whereas I think it
> should be a mask of 0x180. Without that we would only treat the low 8
> bits of R as live, rather than the low 16 bits.
Or I suppose more to the point...
>> +
>> + /* We can certainly get (zero_extract (subreg ...)). The
>> + mode of the zero_extract and location should be sufficient
>> + and we can just strip the SUBREG. */
>> + if (SUBREG_P (x))
>> + x = SUBREG_REG (x);
>> + }
>> +
>> + /* BIT >= 64 indicates something went horribly wrong. */
>> + gcc_assert (bit <= 63);
>> +
>> + /* Now handle the actual object that was changed. */
>> + if (REG_P (x))
>> + {
>> + /* Transfer the appropriate bits from LIVENOW into
>> + LIVE_TMP. */
>> + HOST_WIDE_INT rn = REGNO (x);
>> + for (HOST_WIDE_INT i = 4 * rn; i < 4 * rn + 4; i++)
>> + if (bitmap_bit_p (livenow, i))
>> + bitmap_set_bit (live_tmp, i);
>> +
>> + /* Now clear the bits known written by this instruction.
>> + Note that BIT need not be a power of two, consider a
>> + ZERO_EXTRACT destination. */
>> + int start = (bit < 8 ? 0 : bit < 16 ? 1 : bit < 32 ? 2 : 3);
>> + int end = ((mask & ~0xffffffffULL) ? 4
>> + : (mask & 0xffff0000ULL) ? 3
>> + : (mask & 0xff00) ? 2 : 1);
>> + bitmap_clear_range (livenow, 4 * rn + start, end - start);
...these assumptions don't seem safe for a ZERO_EXTRACT. Unlike for
the other destinations, IIUC we can only clear liveness bits if the
ZERO_EXTRACT writes to *all* of the bits in the range, rather than
some of them. The bits that aren't explicitly written to are preserved.
So...
> [...]
> But for now it might be simpler to remove ZERO_EXTRACT from the "else if"
> and leave a FIXME, since continuing will be conservatively correct.
...I think this applies more generally. It might be safer to handle
ZERO_EXTRACT conservatively for GCC 14 and only try to optimise it in GCC 15.
Thanks,
Richard
