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From: Eduard Zingerman To: Alexei Starovoitov , bpf@vger.kernel.org Cc: daniel@iogearbox.net, andrii@kernel.org, martin.lau@kernel.org, memxor@gmail.com, kernel-team@fb.com Date: Thu, 13 Jun 2024 09:10:22 -0700 In-Reply-To: <20240613013815.953-3-alexei.starovoitov@gmail.com> References: <20240613013815.953-1-alexei.starovoitov@gmail.com> <20240613013815.953-3-alexei.starovoitov@gmail.com> Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.44.4-0ubuntu2 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Wed, 2024-06-12 at 18:38 -0700, Alexei Starovoitov wrote: > From: Alexei Starovoitov >=20 > Compilers can generate the code > r1 =3D r2 > r1 +=3D 0x1 > if r2 < 1000 goto ... > use knowledge of r2 range in subsequent r1 operations >=20 > So remember constant delta between r2 and r1 and update r1 after 'if' con= dition. >=20 > Unfortunately LLVM still uses this pattern for loops with 'can_loop' cons= truct: > for (i =3D 0; i < 1000 && can_loop; i++) >=20 > The "undo" pass was introduced in LLVM > https://reviews.llvm.org/D121937 > to prevent this optimization, but it cannot cover all cases. > Instead of fighting middle end optimizer in BPF backend teach the verifie= r > about this pattern. >=20 > Signed-off-by: Alexei Starovoitov > --- Acked-by: Eduard Zingerman