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Being const + weak breaks with some compilers that constant-propagate
from the weak symbol. This behavior is outside of the specification, but
in LLVM is chosen to match GCC's behavior.
LLVM's implementation was set in this patch:
https://github.com/llvm/llvm-project/commit/f49573d1eedcf1e44893d5a062ac1b72c8419646
A const + weak symbol is set to be weak_odr:
https://llvm.org/docs/LangRef.html
ODR is one definition rule, and given there is one constant definition
constant-propagation is possible. It is possible to get this code to
miscompile with LLVM when applying link time optimization. As compilers
become more aggressive, this is likely to break in more instances.
Move the definition of sample_reg_masks to the conditional part of
perf_regs.h and guard usage with HAVE_PERF_REGS_SUPPORT. This avoids the
weak symbol.
Fix an issue when HAVE_PERF_REGS_SUPPORT isn't defined from patch v1.
In v3, add perf_regs.c for architectures that HAVE_PERF_REGS_SUPPORT but
don't declare sample_regs_masks.
Further notes:
Jiri asked:
"Is this just a precaution or you actualy saw some breakage?"
Ian answered:
"We saw a breakage with clang with thinlto enabled for linking. Our
compiler team had recently seen, and were surprised by, a similar issue
and were able to dig out the weak ODR issue."
Signed-off-by: Ian Rogers <irogers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexey Budankov <alexey.budankov@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: clang-built-linux@googlegroups.com
Cc: Guo Ren <guoren@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: linux-riscv@lists.infradead.org
Cc: Mao Han <han_mao@c-sky.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lore.kernel.org/lkml/20191001003623.255186-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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This patch add basic arch initialization and instruction associate
support for the csky CPU architecture.
E.g.:
$ perf annotate --stdio2
Samples: 161 of event 'cpu-clock:pppH', 4000 Hz, Event count (approx.):
40250000, [percent: local period]
test_4() /usr/lib/perf-test/callchain_test
Percent
Disassembly of section .text:
00008420 <test_4>:
test_4():
subi sp, sp, 4
st.w r8, (sp, 0x0)
mov r8, sp
subi sp, sp, 8
subi r3, r8, 4
movi r2, 0
st.w r2, (r3, 0x0)
↓ br 2e
100.00 14: subi r3, r8, 4
ld.w r2, (r3, 0x0)
subi r3, r8, 8
st.w r2, (r3, 0x0)
subi r3, r8, 4
ld.w r3, (r3, 0x0)
addi r2, r3, 1
subi r3, r8, 4
st.w r2, (r3, 0x0)
2e: subi r3, r8, 4
ld.w r2, (r3, 0x0)
lrw r3, 0x98967f // 8598 <main+0x28>
cmplt r3, r2
↑ bf 14
mov r0, r0
mov r0, r0
mov sp, r8
ld.w r8, (sp, 0x0)
addi sp, sp, 4
← rts
Signed-off-by: Mao Han <han_mao@c-sky.com>
Acked-by: Guo Ren <guoren@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: linux-csky@vger.kernel.org
Link: http://lkml.kernel.org/r/d874d7782d9acdad5d98f2f5c4a6fb26fbe41c5d.1561531557.git.han_mao@c-sky.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
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