[llvm-bugs] [Bug 32392] New: Power i32/i64 atomic acquire load lowers to lwsync, not isync

[via llvm-bugs]

http://bugs.llvm.org/show_bug.cgi?id=32392

            Bug ID: 32392
           Summary: Power i32/i64 atomic acquire load lowers to lwsync,
                    not isync
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: Backend: PowerPC
          Assignee: unassignedbugs at nondot.org
          Reporter: timshen91 at gmail.com
                CC: llvm-bugs at lists.llvm.org

GCC lowers acquire load of i32/i64 to isync, but llvm lowers them to lwsync.

Here's the explanation:
http://www.rdrop.com/~paulmck/scalability/paper/N2745r.2011.03.04a.html

A casually written benchmark also shows that isync approach is faster:

GCC:
Benchmark                      Time(ns)        CPU(ns)     Iterations
---------------------------------------------------------------------
BM_Acquire32/threads:1                2.74           2.74   256555796  
BM_Acquire32/threads:2                1.40           2.80   228185266  
BM_Acquire32/threads:4                1.25           4.98   124786836  
BM_Acquire32/threads:176              0.240          8.17    78115312  
BM_Acquire64/threads:1                5.42           5.41   166593868  
BM_Acquire64/threads:2                2.47           4.07   199450866  
BM_Acquire64/threads:4                1.17           3.22   182357912  
BM_Acquire64/threads:176              0.209          6.76    94368384  

LLVM:
Benchmark                      Time(ns)        CPU(ns)     Iterations
---------------------------------------------------------------------
BM_Acquire32/threads:1                8.45           8.44    70487873  
BM_Acquire32/threads:2                2.75           5.49    93091058  
BM_Acquire32/threads:4                5.56          22.2     31810768  
BM_Acquire32/threads:176              1.14          37.4     17600000  
BM_Acquire64/threads:1                6.71           6.71   100000000  
BM_Acquire64/threads:2                4.01           8.00    63776730  
BM_Acquire64/threads:4                5.57          22.2     31363992  
BM_Acquire64/threads:176              1.41          36.9     17600000

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