Re: [PATCH] drm/drm_vblank.c: avoid unsigned int to signed int cast

Sui Jingfeng <15330273260@xxxxxx> · Mon, 22 May 2023 20:53:36 +0800

Hi,

On 2023/5/22 20:13, Jani Nikula wrote:
On Mon, 22 May 2023, Sui Jingfeng <15330273260@xxxxxx> wrote:
Hi,

On 2023/5/22 19:29, Jani Nikula wrote:
On Thu, 18 May 2023, Sui Jingfeng <15330273260@xxxxxx> wrote:
On 2023/5/17 18:59, David Laight wrote:
From: 15330273260@xxxxxx
Sent: 16 May 2023 18:30

From: Sui Jingfeng <suijingfeng@xxxxxxxxxxx>

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.
In general, do not use unsigned types in arithmethic to avoid negative
values, because most people will be tripped over by integer promotion
rules, and you'll get negative values anyway.

I'll bet most people will be surprised to see what this prints:

#include <stdio.h>
#include <stdint.h>

int main(void)
{
	uint16_t x = 0xffff;
	uint16_t y = 0xffff;
	uint64_t z = x * y;

	printf("0x%016lx\n", z);
	printf("%ld\n", z);
Here, please replace the "%ld\n" with the "%lu\n", then you will see the
difference.

you are casting the variable 'z' to signed value,  "%d" is for printing
signed value, and "%u" is for printing unsigned value.

Your simple code explained exactly why you are still in confusion,
Am I?

Take a look at the values, and explain the math.

I meant the value itself is represent with 2's compliment,

when you print a value with '%ld', then you will get the signed version,

when you print a value with '%lu', then you will get the unsigned version.

The result of a u16*u16 couldn't be negative in math.

But when you using a '%ld' or '%d' to print a unsigned value, then is wrong.

This is also the case which you shouldn't using a int type to store the result of u16*u16.

because when I seen a int type, I will choose '%d' to print it,

when I seen a unsigned int type, I will choose '%u' to print it.

when using a int type as the return type, this could lead people to using '%d' to print

such a value. Then, it generate the confusion as this little test program shows.

BR,
Jani.

that is u16 * u16  can yield a negative value if you use the int as the
return type. Because it overflowed.

	printf("%d\n", x * y);
}

And it's not that different from what you have below. Your patch doesn't
change anything, and doesn't make it any less confusing.

BR,
Jani.

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

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