From: "Alexei Starovoitov" <alexei.starovoitov@gmail.com>
To: "Vineet Gupta" <vineet.gupta@linux.dev>, <ast@kernel.org>,
<daniel@iogearbox.net>, <andrii@kernel.org>, <eddyz87@gmail.com>,
<memxor@gmail.com>
Cc: <martin.lau@linux.dev>, <song@kernel.org>,
<yonghong.song@linux.dev>, <jolsa@kernel.org>,
<emil@etsalapatis.com>, <ihor.solodrai@linux.dev>,
<john.fastabend@gmail.com>, <shuah@kernel.org>,
<bpf@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
<linux-kselftest@vger.kernel.org>
Subject: Re: [PATCH bpf-next v2 03/13] bpf: track low-32 scalar equality across zero-extending movs
Date: Sat, 12 Sep 2026 11:59:51 -0700 [thread overview]
Message-ID: <DLDKI0ZXYI7H.1XQZZE2RCU7IF@gmail.com> (raw)
In-Reply-To: <20260910164635.459558-4-vineet.gupta@linux.dev>
On Thu Sep 10, 2026 at 9:46 AM PDT, Vineet Gupta wrote:
> Linked-scalar equality is full-64-bit only. A 32-bit mov from a source
> with unknown high bits therefore has to drop the relationship, and a later
> narrowing of the source never reaches the destination:
>
> r6 = ... /* full 64-bit unknown */
> w7 = w6 /* 32-bit zero-extending mov */
> if w6 != 0 goto ... /* not taken: r6's low 32 bits are 0 */
> if w7 == 0 goto ... /* not deduced today */
>
> Record a low-32-only link instead: dst shares src's low 32 bits and its
> high half is zero. On a later narrowing, sync_linked_regs() rebuilds such
> a register from the base rather than copying it, by re-applying the same
> zext_32_to_64() the mov used. The reverse direction is skipped: a ->subreg
> base knows nothing about a full register's high half.
at the first glance SUBREG_ZEXT is exactly the same as ADD_CONST32 delta == 0.
no?
Both are unidirectional:
w6->id == 1
w7->id == 1, add_const == 32, delta == 0
will zero extend w7.
This new SUBREG_ZEXT will do the same.
What am I missing?
next prev parent reply other threads:[~2026-09-12 18:59 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-09-10 16:46 [PATCH bpf-next v2 00/13] bpf: track scalar equality across the low 32 bits Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 01/13] bpf: move linked-scalar flags out of bpf_reg_state->id [NFC] Vineet Gupta
2026-09-10 17:52 ` bot+bpf-ci
2026-09-12 18:50 ` Alexei Starovoitov
2026-09-10 16:46 ` [PATCH bpf-next v2 02/13] bpf: compare linked-scalar kinds in regs_exact() Vineet Gupta
2026-09-12 18:51 ` Alexei Starovoitov
2026-09-10 16:46 ` [PATCH bpf-next v2 03/13] bpf: track low-32 scalar equality across zero-extending movs Vineet Gupta
2026-09-10 17:52 ` bot+bpf-ci
2026-09-11 9:29 ` Vineet Gupta
2026-09-12 18:59 ` Alexei Starovoitov [this message]
2026-09-10 16:46 ` [PATCH bpf-next v2 04/13] selftests/bpf: cover the low-32 link for " Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 05/13] bpf: keep the range across a sign extension that cannot change it Vineet Gupta
2026-09-10 17:52 ` bot+bpf-ci
2026-09-11 10:37 ` Vineet Gupta
2026-09-12 19:02 ` Alexei Starovoitov
2026-09-10 16:46 ` [PATCH bpf-next v2 06/13] selftests/bpf: cover sign extensions that cannot change the range Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 07/13] bpf: track low-32 scalar equality across sign-extending movs Vineet Gupta
2026-09-10 17:52 ` bot+bpf-ci
2026-09-11 10:00 ` Vineet Gupta
2026-09-12 19:09 ` Alexei Starovoitov
2026-09-10 16:46 ` [PATCH bpf-next v2 08/13] selftests/bpf: cover the low-32 link for " Vineet Gupta
2026-09-10 17:52 ` bot+bpf-ci
2026-09-11 8:00 ` Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 09/13] bpf: track low-32 scalar equality across narrowing stack fills Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 10/13] selftests/bpf: cover the low-32 link for " Vineet Gupta
2026-09-10 17:31 ` bot+bpf-ci
2026-09-11 5:07 ` Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 11/13] bpf: record what a narrowing spill actually stores Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 12/13] bpf: track low-32 scalar equality across narrowing stack spills Vineet Gupta
2026-09-10 16:46 ` [PATCH bpf-next v2 13/13] selftests/bpf: cover the low-32 link for " Vineet Gupta
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