On Tue, Apr 20, 2021 at 04:50:40AM +0200, Maciej W. Rozycki wrote:
> Our current MIPS platform `__div64_32' handler is inactive, because it
> is incorrectly only enabled for 64-bit configurations, for which generic
> `do_div' code does not call it anyway.
>
> The handler is not suitable for being called from there though as it
> only calculates 32 bits of the quotient under the assumption the 64-bit
> divident has been suitably reduced. Code for such reduction used to be
> there, however it has been incorrectly removed with commit c21004cd5b4c
> ("MIPS: Rewrite <asm/div64.h> to work with gcc 4.4.0."), which should
> have only updated an obsoleted constraint for an inline asm involving
> $hi and $lo register outputs, while possibly wiring the original MIPS
> variant of the `do_div' macro as `__div64_32' handler for the generic
> `do_div' implementation
>
> Correct the handler as follows then:
>
> - Revert most of the commit referred, however retaining the current
> formatting, except for the final two instructions of the inline asm
> sequence, which the original commit missed. Omit the original 64-bit
> parts though.
>
> - Rename the original `do_div' macro to `__div64_32'. Use the combined
> `x' constraint referring to the MD accumulator as a whole, replacing
> the original individual `h' and `l' constraints used for $hi and $lo
> registers respectively, of which `h' has been obsoleted with GCC 4.4.
> Update surrounding code accordingly.
>
> We have since removed support for GCC versions before 4.9, so no need
> for a special arrangement here; GCC has supported the `x' constraint
> since forever anyway, or at least going back to 1991.
>
> - Rename the `__base' local variable in `__div64_32' to `__radix' to
> avoid a conflict with a local variable in `do_div'.
>
> - Actually enable this code for 32-bit rather than 64-bit configurations
> by qualifying it with BITS_PER_LONG being 32 instead of 64. Include
> <asm/bitsperlong.h> for this macro rather than <linux/types.h> as we
> don't need anything else.
>
> - Finally include <asm-generic/div64.h> last rather than first.
>
> This has passed correctness verification with test_div64 and reduced the
> module's average execution time down to 1.0668s and 0.2629s from 2.1529s
> and 0.5647s respectively for an R3400 CPU @40MHz and a 5Kc CPU @160MHz.
> For a reference 64-bit `do_div' code where we have the DDIVU instruction
> available to do the whole calculation right away averages at 0.0660s for
> the latter CPU.
>
This patch results in:
arch/mips/mti-malta/malta-time.c: In function 'plat_time_init':
./arch/mips/include/asm/div64.h:76:3: error: inconsistent operand constraints in an 'asm'
and similar errors when trying to compile malta_qemu_32r6_defconfig.
I tried with gcc 8.3.0, 8.4.0, 9.3.0, and 10.3.0.
Does this need some additional new compile flags ?
Thanks,
Guenter
