linux-kernel - Re: [PATCH] radeon: Shrink radeon_ring

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Date:	Mon, 18 May 2015 20:25:48 +0200
From:	Denys Vlasenko <dvlasenk@...hat.com>
To:	Christian König <christian.koenig@....com>
CC:	Alex Deucher <alexander.deucher@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] radeon: Shrink radeon_ring_write()

On 05/18/2015 08:11 PM, Christian König wrote:
> De-duplicating the error message is probably a good idea,
> but are the remaining code changes really necessary for the size reduction?

The conversion from

   if (ring->count_dw <= 0)
        error;
   ...
   ring->count_dw--;

to

   if (--ring->count_dw < 0)
        error;
   ...

eliminates one access to ring->count_dw.


Conversion from

    ring->ring[ring->wptr++] = v;
    ring->wptr &= ring->ptr_mask;

to

    ring->ring[ring->wptr] = v;
    ring->wptr = (ring->wptr + 1) & ring->ptr_mask;

ideally (with infinitely smart compiler)
shouldn't be necessary, but at least for my gcc
version it eliminates one additional insn:
gcc copies ring->wptr to a scratch reg, then increments it,
then uses scratch reg for addressing...


> On 18.05.2015 19:59, Denys Vlasenko wrote:
>> Inlined radeon_ring_write() has 729 callers, which amounts to about 50000
>> bytes of code. however, deinlining it is probably too much
>> of a performance impact.
>>
>> This patch shrinks slow path a bit and optimizes fast path.
>> Comparison of generated machine code is below:
>>
>> old___________________________________ new____________________________
>> 55             push %rbp               55         push %rbp
>> 4889e5         mov  %rsp,%rbp          ff4f38     decl 0x38(%rdi)
>> 4154           push %r12               4889e5     mov  %rsp,%rbp
>> 4189f4         mov  %esi,%r12d         4154       push %r12
>> 53             push %rbx               4189f4     mov  %esi,%r12d
>> 837f3800       cmpl $0x0,0x38(%rdi)    53         push %rbx
>> 4889fb         mov  %rdi,%rbx          4889fb     mov  %rdi,%rbx
>> 7f0e           jg   <.Lbl>             7905       jns  <.Lbl>
>> 48c7c78f51a785 mov  $message,%rdi
>> 31c0           xor  %eax,%eax
>> e89306f9ff     call <drm_err>          e8cbffffff call <radeon_ring_overflow>
>>      .Lbl:
>> 8b4328         mov  0x28(%rbx),%eax    8b5328     mov  0x28(%rbx),%edx
>> 488b5308       mov  0x8(%rbx),%rdx     488b4308   mov  0x8(%rbx),%rax
>> 89c1           mov  %eax,%ecx          488d0490   lea  (%rax,%rdx,4),%rax
>> ffc0           inc  %eax
>> 488d148a       lea  (%rdx,%rcx,4),%rdx 448920     mov  %r12d,(%rax)
>> 448922         mov  %r12d,(%rdx)       8b4328     mov  0x28(%rbx),%eax
>> 234354         and  0x54(%rbx),%eax    ff4b34     decl 0x34(%rbx)
>> ff4b38         decl 0x38(%rbx)         ffc0       inc  %eax
>> ff4b34         decl 0x34(%rbx)         234354     and  0x54(%rbx),%eax
>> 894328         mov  %eax,0x28(%rbx)    894328     mov  %eax,0x28(%rbx)
>> 5b             pop  %rbx               5b         pop  %rbx
>> 415c           pop  %r12               415c       pop  %r12
>> 5d             pop  %rbp               5d         pop  %rbp
>>
>> This shaves off more than 10 kbytes of code off the kernel:
>>
>>      text     data      bss       dec     hex filename
>> 85657104 22294872 20627456 128579432 7a9f768 vmlinux.before
>> 85646544 22294872 20627456 128568872 7a9ce28 vmlinux
>>
>> Signed-off-by: Denys Vlasenko <dvlasenk@...hat.com>
>> Cc: Christian König <christian.koenig@....com>
>> Cc: Alex Deucher <alexander.deucher@....com>
>> Cc: linux-kernel@...r.kernel.org
>> ---
>>   drivers/gpu/drm/radeon/radeon.h      | 11 +++++------
>>   drivers/gpu/drm/radeon/radeon_ring.c |  5 +++++
>>   2 files changed, 10 insertions(+), 6 deletions(-)
>>
>> diff --git a/drivers/gpu/drm/radeon/radeon.h b/drivers/gpu/drm/radeon/radeon.h
>> index bb6b25c..9106873 100644
>> --- a/drivers/gpu/drm/radeon/radeon.h
>> +++ b/drivers/gpu/drm/radeon/radeon.h
>> @@ -2658,14 +2658,13 @@ void radeon_atombios_fini(struct radeon_device *rdev);
>>    *
>>    * Write a value to the requested ring buffer (all asics).
>>    */
>> +void radeon_ring_overflow(void);
>>   static inline void radeon_ring_write(struct radeon_ring *ring, uint32_t v)
>>   {
>> -    if (ring->count_dw <= 0)
>> -        DRM_ERROR("radeon: writing more dwords to the ring than expected!\n");
>> -
>> -    ring->ring[ring->wptr++] = v;
>> -    ring->wptr &= ring->ptr_mask;
>> -    ring->count_dw--;
>> +    if (--ring->count_dw < 0)
>> +        radeon_ring_overflow();
>> +    ring->ring[ring->wptr] = v;
>> +    ring->wptr = (ring->wptr + 1) & ring->ptr_mask;
>>       ring->ring_free_dw--;
>>   }
>>   diff --git a/drivers/gpu/drm/radeon/radeon_ring.c b/drivers/gpu/drm/radeon/radeon_ring.c
>> index 2456f69..8204c23 100644
>> --- a/drivers/gpu/drm/radeon/radeon_ring.c
>> +++ b/drivers/gpu/drm/radeon/radeon_ring.c
>> @@ -126,6 +126,11 @@ int radeon_ring_alloc(struct radeon_device *rdev, struct radeon_ring *ring, unsi
>>       return 0;
>>   }
>>   +void radeon_ring_overflow(void)
>> +{
>> +    DRM_ERROR("radeon: writing more dwords to the ring than expected!\n");
>> +}
>> +
>>   /**
>>    * radeon_ring_lock - lock the ring and allocate space on it
>>    *
> 

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