On 6/16/14, 3:00 PM, Phoebe Buckheister wrote:
Current code to determine availability of atomics seems a bit
incorrect, or at least weird.
I don't quite understand why clang looks at the OS specified in the
triple, as availability of atomic instructions doesn't depend on the
OS; it only depends on the arch version and the execution mode. Atomics
are there in ARM mode on v6 and later, and in Thumb mode on v7 and later
(but no ldrexd/strexd in the M profile).
Clang is looking at the OS because these OSes support calling a kernel provided
cmpxchg, which is what you need to make the __sync_* builtins work on pre v6
hardware (and this is how they are implemented in libgcc).
Given that, I think that this check and comment:
```
- // On linux, binaries targeting old cpus call functions in libgcc to
- // perform atomic operations. The implementation in libgcc then calls into
- // the kernel which on armv6 and newer uses ldrex and strex. The net result
- // is that if we assume the kernel is at least as recent as the hardware,
- // it is safe to use atomic instructions on armv6 and newer.
- if (!T.isOSLinux() &&
- T.getOS() != llvm::Triple::FreeBSD &&
- T.getOS() != llvm::Triple::NetBSD &&
- T.getOS() != llvm::Triple::Bitrig)
- return false;
```
ought to really be placed as a fallback if the HW is older than v6.
Cheers,
Jon
This patch is an attempt to rectify the situation: check only the arch
version - correctly, as thumbv7em fails the old check - and allow
atomics up to 32 bits on the ARMv7 M profiles.
If the OS check has a reason I missed, I'll add it back - but bare
metal wants atomics too ...
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Jon Roelofs
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CodeSourcery / Mentor Embedded
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