Minki/linux
1
0
Fork 1
mirror of https://github.com/torvalds/linux.git synced 2024-11-10 22:21:40 +00:00
Code Issues Packages Projects Releases Wiki Activity

lib: zstd: Add cast to silence clang's -Wbitwise-instead-of-logical

Browse Source 
A new warning in clang warns that there is an instance where boolean
expressions are being used with bitwise operators instead of logical
ones:

lib/zstd/decompress/huf_decompress.c:890:25: warning: use of bitwise '&' with boolean operands [-Wbitwise-instead-of-logical]
                       (BIT_reloadDStreamFast(&bitD1) == BIT_DStream_unfinished)
                       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

zstd does this frequently to help with performance, as logical operators
have branches whereas bitwise ones do not.

To fix this warning in other cases, the expressions were placed on
separate lines with the '&=' operator; however, this particular instance
was moved away from that so that it could be surrounded by LIKELY, which
is a macro for __builtin_expect(), to help with a performance
regression, according to upstream zstd pull #1973.

Aside from switching to logical operators, which is likely undesirable
in this instance, or disabling the warning outright, the solution is
casting one of the expressions to an integer type to make it clear to
clang that the author knows what they are doing. Add a cast to U32 to
silence the warning. The first U32 cast is to silence an instance of
-Wshorten-64-to-32 because __builtin_expect() returns long so it cannot
be moved.

Link: https://github.com/ClangBuiltLinux/linux/issues/1486
Link: https://github.com/facebook/zstd/pull/1973
Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Nick Terrell <terrelln@fb.com>
This commit is contained in:
Nathan Chancellor 2021-10-21 13:23:53 -07:00 committed by Nick Terrell
parent a99a65cfb9
commit 0a8ea23583
1 changed files with 1 additions and 1 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
lib/zstd/decompress/huf_decompress.c
View File

@ -886,7 +886,7 @@ HUF_decompress4X2_usingDTable_internal_body(
HUF_DECODE_SYMBOLX2_0(op2, &bitD2);
HUF_DECODE_SYMBOLX2_0(op3, &bitD3);
HUF_DECODE_SYMBOLX2_0(op4, &bitD4);
endSignal = (U32)LIKELY(
endSignal = (U32)LIKELY((U32)
(BIT_reloadDStreamFast(&bitD1) == BIT_DStream_unfinished)
& (BIT_reloadDStreamFast(&bitD2) == BIT_DStream_unfinished)
& (BIT_reloadDStreamFast(&bitD3) == BIT_DStream_unfinished)