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Subject: Re: [PATCH] drm/drm_vblank.c: avoid unsigned int to signed int cast
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To:     David Laight <David.Laight@ACULAB.COM>,
        Sui Jingfeng <suijingfeng@loongson.cn>,
        Li Yi <liyi@loongson.cn>
Cc:     Thomas Zimmermann <tzimmermann@suse.de>,
        Maarten Lankhorst <maarten.lankhorst@linux.intel.com>,
        Maxime Ripard <mripard@kernel.org>,
        David Airlie <airlied@gmail.com>,
        Daniel Vetter <daniel@ffwll.ch>,
        "dri-devel@lists.freedesktop.org" <dri-devel@lists.freedesktop.org>,
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        <loongson-kernel@lists.loongnix.cn>
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From:   Sui Jingfeng <15330273260@189.cn>
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On 2023/5/17 18:59, David Laight wrote:
> From: 15330273260@189.cn
>> Sent: 16 May 2023 18:30
>>
>> From: Sui Jingfeng <suijingfeng@loongson.cn>
>>
>> Both mode->crtc_htotal and mode->crtc_vtotal are u16 type,
>> mode->crtc_htotal * mode->crtc_vtotal will results a unsigned type.
> Nope, u16 gets promoted to 'signed int' and the result of the
> multiply is also signed.

I believe that signed or unsigned is dependent on the declaration.

I am talk about the math, while you are talking about compiler.

I admit that u16 gets promoted to 'signed int' is true, but this is 
irrelevant,

the point is how to understand the returned value.


How does the compiler generate the code is one thing, how do we 
interpret the result is another

How does the compiler generate the code is NOT determined by us, while 
how do we interpret the result is determined by us.


I believe that using a u32 type to interpret the result(u16 * u16) is 
always true, it is true in the perspective of *math*.

Integer promotions is the details of C program language. If the result 
of the multiply is signed, then there are risks that

the result is negative, what's the benefit to present this risk to the 
programmer?

What's the benefit to tell me(and others) that u16 * u16 yield a signed 
value? and can be negative?

Using int type as the return type bring concerns to the programmer and 
the user of the function,

even though this is not impossible in practice.

>> Using a u32 is enough to store the result, but considering that the
>> result will be casted to u64 soon after. We use a u64 type directly.
>> So there no need to cast it to signed type and cast back then.
> ....
>> -		int frame_size = mode->crtc_htotal * mode->crtc_vtotal;
>> +		u64 frame_size = mode->crtc_htotal * mode->crtc_vtotal;
> ...
>> -		framedur_ns = div_u64((u64) frame_size * 1000000, dotclock);
>> +		framedur_ns = div_u64(frame_size * 1000000, dotclock);
> The (u64) cast is there to extend the value to 64bits, not
> because the original type is signed.

Sorry about my expression, I think my sentence did not mention anything 
about 'because the original type is signed'.

In the contrary, my patch eliminated the concerns to the reviewer. It 
say that the results of the multiply can't be negative.

My intent is to tell the compiler we want a unsigned return type, but 
GCC emit 'imul' instruction for the multiply......

I'm using u64 as the return type, because div_u64() function accept a 
u64 type value as its first argument.

> The compiler will detect that the old code is a 32x32 multiply
> where a 64bit result is needed, that may not be true for the
> changed code (it would need to track back as far as the u16s).

I don't believe my code could be wrong.

when you use the word 'may', you are saying that it could be wrong after 
apply my patch.

Then you have to find at least one test example to prove you point, in 
which case my codes generate wrong results.

Again I don't believe you could find one.

> It is not uncommon to force a 64bit result from a multiply
> by making the constant 64bit. As in:
> 	div_u64(frame_size * 1000000ULL, dotclock);

In fact, After apply this patch, the ASM code generated is same with before.

This may because the GCC is smart enough to generate optimized code in 
either case,

I think It could be different with a different optimization-level.

I have tested this patch on three different architecture,  I can not 
find error still.

Below is the assembly extract on x86-64: because GCC generate the same 
code in either case,

so I pasted only one copy here.


0000000000000530 <drm_calc_timestamping_constants>:
      530:    f3 0f 1e fa              endbr64
      534:    e8 00 00 00 00           callq  539 
<drm_calc_timestamping_constants+0x9>
      539:    55                       push   %rbp
      53a:    48 89 e5                 mov    %rsp,%rbp
      53d:    41 57                    push   %r15
      53f:    41 56                    push   %r14
      541:    41 55                    push   %r13
      543:    41 54                    push   %r12
      545:    53                       push   %rbx
      546:    48 83 ec 18              sub    $0x18,%rsp
      54a:    4c 8b 3f                 mov    (%rdi),%r15
      54d:    41 8b 87 6c 01 00 00     mov    0x16c(%r15),%eax
      554:    85 c0                    test   %eax,%eax
      556:    0f 84 ec 00 00 00        je     648 
<drm_calc_timestamping_constants+0x118>
      55c:    44 8b 87 90 00 00 00     mov    0x90(%rdi),%r8d
      563:    49 89 fc                 mov    %rdi,%r12
      566:    44 39 c0                 cmp    %r8d,%eax
      569:    0f 86 40 01 00 00        jbe    6af 
<drm_calc_timestamping_constants+0x17f>
      56f:    44 8b 76 1c              mov    0x1c(%rsi),%r14d
      573:    49 8b 8f 40 01 00 00     mov    0x140(%r15),%rcx
      57a:    48 89 f3                 mov    %rsi,%rbx
      57d:    45 85 f6                 test   %r14d,%r14d
      580:    0f 8e d5 00 00 00        jle    65b 
<drm_calc_timestamping_constants+0x12b>
      586:    0f b7 43 2a              movzwl 0x2a(%rbx),%eax
      58a:    49 63 f6                 movslq %r14d,%rsi
      58d:    31 d2                    xor    %edx,%edx
      58f:    48 89 c7                 mov    %rax,%rdi
      592:    48 69 c0 40 42 0f 00     imul   $0xf4240,%rax,%rax
      599:    48 f7 f6                 div    %rsi
      59c:    31 d2                    xor    %edx,%edx
      59e:    48 89 45 d0              mov    %rax,-0x30(%rbp)
      5a2:    0f b7 43 38              movzwl 0x38(%rbx),%eax
      5a6:    0f af c7                 imul   %edi,%eax
      5a9:    48 98                    cltq
      5ab:    48 69 c0 40 42 0f 00     imul   $0xf4240,%rax,%rax
      5b2:    48 f7 f6                 div    %rsi
      5b5:    41 89 c5                 mov    %eax,%r13d
      5b8:    f6 43 18 10              testb  $0x10,0x18(%rbx)
      5bc:    74 0a                    je     5c8 
<drm_calc_timestamping_constants+0x98>
      5be:    41 c1 ed 1f              shr    $0x1f,%r13d
      5c2:    41 01 c5                 add    %eax,%r13d
      5c5:    41 d1 fd                 sar    %r13d
      5c8:    4b 8d 04 c0              lea    (%r8,%r8,8),%rax
      5cc:    48 89 de                 mov    %rbx,%rsi
      5cf:    49 8d 3c 40              lea    (%r8,%rax,2),%rdi
      5d3:    8b 45 d0                 mov    -0x30(%rbp),%eax
      5d6:    48 c1 e7 04              shl    $0x4,%rdi
      5da:    48 01 cf                 add    %rcx,%rdi
      5dd:    89 47 78                 mov    %eax,0x78(%rdi)
      5e0:    48 83 ef 80              sub $0xffffffffffffff80,%rdi
      5e4:    44 89 6f f4              mov    %r13d,-0xc(%rdi)
      5e8:    e8 00 00 00 00           callq  5ed 
<drm_calc_timestamping_constants+0xbd>
      5ed:    0f b7 53 2e              movzwl 0x2e(%rbx),%edx
      5f1:    0f b7 43 38              movzwl 0x38(%rbx),%eax
      5f5:    44 0f b7 4b 2a           movzwl 0x2a(%rbx),%r9d
      5fa:    45 8b 44 24 60           mov    0x60(%r12),%r8d
      5ff:    4d 85 ff                 test   %r15,%r15
      602:    0f 84 87 00 00 00        je     68f 
<drm_calc_timestamping_constants+0x15f>
      608:    49 8b 77 08              mov    0x8(%r15),%rsi
      60c:    52                       push   %rdx
      60d:    31 ff                    xor    %edi,%edi
      60f:    48 c7 c1 00 00 00 00     mov    $0x0,%rcx
      616:    50                       push   %rax
      617:    31 d2                    xor    %edx,%edx
      619:    e8 00 00 00 00           callq  61e 
<drm_calc_timestamping_constants+0xee>
      61e:    45 8b 44 24 60           mov    0x60(%r12),%r8d
      623:    4d 8b 7f 08              mov    0x8(%r15),%r15
      627:    5f                       pop    %rdi
      628:    41 59                    pop    %r9
      62a:    8b 45 d0                 mov    -0x30(%rbp),%eax
      62d:    48 c7 c1 00 00 00 00     mov    $0x0,%rcx
      634:    4c 89 fe                 mov    %r15,%rsi
      637:    45 89 f1                 mov    %r14d,%r9d
      63a:    31 d2                    xor    %edx,%edx
      63c:    31 ff                    xor    %edi,%edi
      63e:    50                       push   %rax
      63f:    41 55                    push   %r13
      641:    e8 00 00 00 00           callq  646 
<drm_calc_timestamping_constants+0x116>
      646:    59                       pop    %rcx
      647:    5e                       pop    %rsi
      648:    48 8d 65 d8              lea    -0x28(%rbp),%rsp
      64c:    5b                       pop    %rbx
      64d:    41 5c                    pop    %r12
      64f:    41 5d                    pop    %r13
      651:    41 5e                    pop    %r14
      653:    41 5f                    pop    %r15
      655:    5d                       pop    %rbp
      656:    e9 00 00 00 00           jmpq   65b 
<drm_calc_timestamping_constants+0x12b>
      65b:    41 8b 54 24 60           mov    0x60(%r12),%edx
      660:    49 8b 7f 08              mov    0x8(%r15),%rdi
      664:    44 89 45 c4              mov    %r8d,-0x3c(%rbp)
      668:    45 31 ed                 xor    %r13d,%r13d
      66b:    48 c7 c6 00 00 00 00     mov    $0x0,%rsi
      672:    48 89 4d c8              mov    %rcx,-0x38(%rbp)
      676:    e8 00 00 00 00           callq  67b 
<drm_calc_timestamping_constants+0x14b>
      67b:    c7 45 d0 00 00 00 00     movl   $0x0,-0x30(%rbp)
      682:    44 8b 45 c4              mov    -0x3c(%rbp),%r8d
      686:    48 8b 4d c8              mov    -0x38(%rbp),%rcx
      68a:    e9 39 ff ff ff           jmpq   5c8 
<drm_calc_timestamping_constants+0x98>
      68f:    52                       push   %rdx
      690:    48 c7 c1 00 00 00 00     mov    $0x0,%rcx
      697:    31 d2                    xor    %edx,%edx
      699:    31 f6                    xor    %esi,%esi
      69b:    50                       push   %rax
      69c:    31 ff                    xor    %edi,%edi
      69e:    e8 00 00 00 00           callq  6a3 
<drm_calc_timestamping_constants+0x173>
      6a3:    45 8b 44 24 60           mov    0x60(%r12),%r8d
      6a8:    58                       pop    %rax
      6a9:    5a                       pop    %rdx
      6aa:    e9 7b ff ff ff           jmpq   62a 
<drm_calc_timestamping_constants+0xfa>
      6af:    49 8b 7f 08              mov    0x8(%r15),%rdi
      6b3:    4c 8b 67 50              mov    0x50(%rdi),%r12
      6b7:    4d 85 e4                 test   %r12,%r12
      6ba:    74 25                    je     6e1 
<drm_calc_timestamping_constants+0x1b1>
      6bc:    e8 00 00 00 00           callq  6c1 
<drm_calc_timestamping_constants+0x191>
      6c1:    48 c7 c1 00 00 00 00     mov    $0x0,%rcx
      6c8:    4c 89 e2                 mov    %r12,%rdx
      6cb:    48 c7 c7 00 00 00 00     mov    $0x0,%rdi
      6d2:    48 89 c6                 mov    %rax,%rsi
      6d5:    e8 00 00 00 00           callq  6da 
<drm_calc_timestamping_constants+0x1aa>
      6da:    0f 0b                    ud2
      6dc:    e9 67 ff ff ff           jmpq   648 
<drm_calc_timestamping_constants+0x118>
      6e1:    4c 8b 27                 mov    (%rdi),%r12
      6e4:    eb d6                    jmp    6bc 
<drm_calc_timestamping_constants+0x18c>
      6e6:    66 2e 0f 1f 84 00 00     nopw   %cs:0x0(%rax,%rax,1)
      6ed:    00 00 00
      6f0:    90                       nop
      6f1:    90                       nop
      6f2:    90                       nop
      6f3:    90                       nop
      6f4:    90                       nop
      6f5:    90                       nop
      6f6:    90                       nop
      6f7:    90                       nop
      6f8:    90                       nop
      6f9:    90                       nop
      6fa:    90                       nop
      6fb:    90                       nop
      6fc:    90                       nop
      6fd:    90                       nop
      6fe:    90                       nop
      6ff:    90                       nop


> 	David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> Registration No: 1397386 (Wales)
>