On 7/12/24 11:30 AM, Alexei Starovoitov wrote:
On Thu, Jul 11, 2024 at 10:07 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
Here we would like to handle a special case after sign extending load,
when upper bits for a 64-bit range are all 1s or all 0s.
Upper bits are all 1s when register is in a rage:
[0xffff_ffff_0000_0000, 0xffff_ffff_ffff_ffff]
Upper bits are all 0s when register is in a range:
[0x0000_0000_0000_0000, 0x0000_0000_ffff_ffff]
Together this forms are continuous range:
[0xffff_ffff_0000_0000, 0x0000_0000_ffff_ffff]
Now, suppose that register range is in fact tighter:
[0xffff_ffff_8000_0000, 0x0000_0000_ffff_ffff] (R)
Also suppose that it's 32-bit range is positive,
meaning that lower 32-bits of the full 64-bit register
are in the range:
[0x0000_0000, 0x7fff_ffff] (W)
It so happens, that any value in a range:
[0xffff_ffff_0000_0000, 0xffff_ffff_7fff_ffff]
is smaller than a lowest bound of the range (R):
0xffff_ffff_8000_0000
which means that upper bits of the full 64-bit register
can't be all 1s, when lower bits are in range (W).
Note that:
- 0xffff_ffff_8000_0000 == (s64)S32_MIN
- 0x0000_0000_ffff_ffff == (s64)S32_MAX
These relations are used in the conditions below.
Sounds good. I will add some comments like the above in v2.
I would add Ed's explanation verbatim as a comment to verifier.c
+ if (reg->s32_min_value >= 0) {
+ if ((reg->smin_value == S32_MIN && reg->smax_value <= S32_MAX) ||
+ (reg->smin_value == S16_MIN && reg->smax_value <= S16_MAX) ||
+ (reg->smin_value == S8_MIN && reg->smax_value <= S8_MAX)) {
The explanation above also lands a question, would it be correct to
replace the checks above by a single one?
reg->smin_value >= S32_MIN && reg->smax_value <= S32_MAX
You are correct, the range check can be better. The following is the related
description in the commit message:
This patch fixed the issue by adding additional register deduction after 32-bit compare
insn such that if the signed 32-bit register range is non-negative and 64-bit smin is
{S32/S16/S8}_MIN and 64-bit max is no greater than {U32/U16/U8}_MAX.
Here, we check smin with {S32/S16/S8}_MIN since this is the most common result related to
signed extension load.
The corrent code simply represents the most common pattern.
Since you mention this, I will resive it as below in v2:
reg->smin_value >= S32_MIN && reg->smin_value < 0 && reg->smax_value <= S32_MAX
Why add smin_value < 0 check ?
I'd think
if (reg->s32_min_value >= 0 && reg->smin_value >= S32_MIN &&
reg->smax_value <= S32_MAX)
is enough?
This is enough and correct. As you mentioned below, if smin_value >= 0 it is just
a redundant work but no hurt.
I will do as you suggested.
If smin_value is >=0 it's fine to reassign it with s32_min_value
which is positive as well.
