[PATCH] IR support for "cmpxchg weak"

[Hal Finkel]

----- Original Message -----
> From: "Tim Northover" <t.p.northover at gmail.com>
> To: "t p northover" <t.p.northover at gmail.com>
> Cc: llvm-commits at cs.uiuc.edu
> Sent: Friday, June 13, 2014 8:09:33 AM
> Subject: [PATCH] IR support for "cmpxchg weak"
> 
> Hi all,
> 
> Given the size & scope of this change, it's definitely worth getting
> the details checked over as much as possible. The patch here is
> substantially the same as the one in the RFC thread, but tidied up
> and with a bit more testing.
> 
> This patch is minimal, in the sense that it exposes some interfaces
> to make "cmpxchg weak" usable, but doesn't actually do anything with
> them. It comes with a similarly minimal Clang patch that just makes
> it correctly use the strong cmpxchg again.

This somewhat concerns me... does this make 'weak' the new default? If that is not backward compatible then maybe we should flip the default?

 -Hal

> 
> I'm expecting 3 following patches, though they'll all be fairly
> trivial and obvious once this is in place:
> + Use the "weak" in AtomicExpandLoadLinked. This gives us actual
> benefit from the construct.
> + Use the i1 in X86ISelLowering.cpp. This lets us skip some redundant
> comparisons.
> + Make clang emit "cmpxchg weak" when appropriate. It's almost all
> wired up already; just a few extra lines and a bunch of tests.
> 
> OK to commit?
> 
> Tim.
> 
> http://reviews.llvm.org/D4134
> 
> Files:
>   docs/Atomics.rst
>   docs/LangRef.rst
>   include/llvm/CodeGen/ISDOpcodes.h
>   include/llvm/CodeGen/SelectionDAG.h
>   include/llvm/CodeGen/SelectionDAGNodes.h
>   include/llvm/IR/Instructions.h
>   lib/AsmParser/LLParser.cpp
>   lib/Bitcode/Reader/BitcodeReader.cpp
>   lib/Bitcode/Writer/BitcodeWriter.cpp
>   lib/CodeGen/AtomicExpandLoadLinkedPass.cpp
>   lib/CodeGen/SelectionDAG/LegalizeDAG.cpp
>   lib/CodeGen/SelectionDAG/LegalizeIntegerTypes.cpp
>   lib/CodeGen/SelectionDAG/LegalizeTypes.h
>   lib/CodeGen/SelectionDAG/SelectionDAG.cpp
>   lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp
>   lib/CodeGen/SelectionDAG/SelectionDAGDumper.cpp
>   lib/CodeGen/TargetLoweringBase.cpp
>   lib/IR/AsmWriter.cpp
>   lib/IR/Instruction.cpp
>   lib/IR/Instructions.cpp
>   lib/Target/CppBackend/CPPBackend.cpp
>   lib/Target/X86/X86ISelLowering.cpp
>   lib/Transforms/IPO/MergeFunctions.cpp
>   lib/Transforms/Instrumentation/ThreadSanitizer.cpp
>   lib/Transforms/Scalar/LowerAtomic.cpp
>   test/Assembler/atomic.ll
>   test/Bitcode/atomic.ll
>   test/Bitcode/memInstructions.3.2.ll
>   test/Bitcode/weak-cmpxchg-upgrade.ll
>   test/Bitcode/weak-cmpxchg-upgrade.ll.bc
>   test/CodeGen/AArch64/arm64-atomic-128.ll
>   test/CodeGen/AArch64/arm64-atomic.ll
>   test/CodeGen/AArch64/atomic-ops.ll
>   test/CodeGen/AArch64/cmpxchg-idioms.ll
>   test/CodeGen/ARM/atomic-64bit.ll
>   test/CodeGen/ARM/atomic-cmp.ll
>   test/CodeGen/ARM/atomic-op.ll
>   test/CodeGen/ARM/atomic-ops-v8.ll
>   test/CodeGen/ARM/cmpxchg-idioms.ll
>   test/CodeGen/CPP/atomic.ll
>   test/CodeGen/Mips/atomic.ll
>   test/CodeGen/Mips/atomicops.ll
>   test/CodeGen/PowerPC/Atomics-32.ll
>   test/CodeGen/PowerPC/atomic-1.ll
>   test/CodeGen/PowerPC/atomic-2.ll
>   test/CodeGen/R600/atomic_cmp_swap_local.ll
>   test/CodeGen/SPARC/atomics.ll
>   test/CodeGen/SystemZ/cmpxchg-01.ll
>   test/CodeGen/SystemZ/cmpxchg-02.ll
>   test/CodeGen/SystemZ/cmpxchg-03.ll
>   test/CodeGen/SystemZ/cmpxchg-04.ll
>   test/CodeGen/X86/2010-10-08-cmpxchg8b.ll
>   test/CodeGen/X86/Atomics-64.ll
>   test/CodeGen/X86/atomic_op.ll
>   test/CodeGen/X86/coalescer-remat.ll
>   test/Instrumentation/MemorySanitizer/atomics.ll
>   test/Transforms/AtomicExpandLoadLinked/ARM/atomic-expansion-v7.ll
>   test/Transforms/AtomicExpandLoadLinked/ARM/atomic-expansion-v8.ll
>   test/Transforms/LowerAtomic/atomic-swap.ll
> 
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-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory