[llvm-bugs] [Bug 34347] New: llvm doesn't generate inline sequence for load/store of unaligned atomic variable on x86_64
via llvm-bugs
llvm-bugs at lists.llvm.org
Mon Aug 28 10:22:57 PDT 2017
https://bugs.llvm.org/show_bug.cgi?id=34347
Bug ID: 34347
Summary: llvm doesn't generate inline sequence for load/store
of unaligned atomic variable on x86_64
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Common Code Generator Code
Assignee: unassignedbugs at nondot.org
Reporter: wmi at google.com
CC: llvm-bugs at lists.llvm.org
-------------------- 1.cc -----------------------
#include <atomic>
struct AM {
int f1;
int f2;
};
std::atomic<AM> ip3{AM()};
std::atomic<AM> ip4{AM()};
void foo() {
auto val = ip4.load(std::memory_order_relaxed);
ip3.store(val, std::memory_order_relaxed);
}
-------------------------------------------------
~/workarea/llvm-r309240/dbuild/bin/clang -O2 -std=c++11 -fno-exceptions -S 1.cc
.cfi_startproc
# BB#0: # %entry
pushq %rax
.Lcfi0:
.cfi_def_cfa_offset 16
movl $local_ip4, %edi
xorl %esi, %esi
callq __atomic_load_8
movl $local_ip3, %edi
xorl %edx, %edx
movq %rax, %rsi
popq %rax
jmp __atomic_store_8 # TAILCALL
.Lfunc_end0:
.size _Z3foov, .Lfunc_end0-_Z3foov
.cfi_endproc
g++ -O2 -std=c++11 -fno-exceptions -S 1.cc
.cfi_startproc
movq ip4(%rip), %rax
movq %rax, ip3(%rip)
ret
.cfi_endproc
If alignas(8) is added:
struct alignas(8) AM {
int f1;
int f2;
};
clang will generate inline sequence for the atomic accesses:
.cfi_startproc
movq ip4(%rip), %rax
movq %rax, ip3(%rip)
retq
--
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