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LLVM commit [1] changed loop pragma behavior such that
full loop unroll is always honored with user pragma.
Previously, unroll count also depends on the unrolled
code size. For pyperf600, without [1], the loop unroll
count is 150. With [1], the loop unroll count is 600.
The unroll count of 600 caused the program size close to
298k and this caused the following code is generated:
0: 7b 1a 00 ff 00 00 00 00 *(u64 *)(r10 - 256) = r1
; uint64_t pid_tgid = bpf_get_current_pid_tgid();
1: 85 00 00 00 0e 00 00 00 call 14
2: bf 06 00 00 00 00 00 00 r6 = r0
; pid_t pid = (pid_t)(pid_tgid >> 32);
3: bf 61 00 00 00 00 00 00 r1 = r6
4: 77 01 00 00 20 00 00 00 r1 >>= 32
5: 63 1a fc ff 00 00 00 00 *(u32 *)(r10 - 4) = r1
6: bf a2 00 00 00 00 00 00 r2 = r10
7: 07 02 00 00 fc ff ff ff r2 += -4
; PidData* pidData = bpf_map_lookup_elem(&pidmap, &pid);
8: 18 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 r1 = 0 ll
10: 85 00 00 00 01 00 00 00 call 1
11: bf 08 00 00 00 00 00 00 r8 = r0
; if (!pidData)
12: 15 08 15 e8 00 00 00 00 if r8 == 0 goto -6123 <LBB0_27588+0xffffffffffdae100>
Note that insn 12 has a branch offset -6123 which is clearly illegal
and will be rejected by the verifier. The negative offset is due to
the branch range is greater than INT16_MAX.
This patch changed the unroll count to be 150 to avoid above
branch target insn out-of-range issue. Also the llvm is enhanced ([2])
to assert if the branch target insn is out of INT16 range.
[1] https://reviews.llvm.org/D119148
[2] https://reviews.llvm.org/D123877
Signed-off-by: Yonghong Song <yhs@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220419043230.2928530-1-yhs@fb.com
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Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.