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Hi all,<br>
<br>
Attached is a sequence of patches that changes the IR to support
more than two synchronization scopes. This is still a work in
progress, and these patches are only meant to start a more detailed
discussion on the way forward.<br>
<br>
One big issue is the absence of any backend that actually makes use
of intermediate synchronization scopes. This work is meant to be
just one part of the ground work required for landing the
much-anticipated HSAIL backend. Also, more work might be needed for
emitting atomic instructions via Clang.<br>
<br>
The proposed syntax for synchronization scope is as follows:<br>
<ol>
<li>Synchronization scopes are of arbitrary width, but implemented
as unsigned in the bitcode, just like address spaces.</li>
<li>Cross-thread is default, but now encoded as 0.<br>
</li>
<li>Keyword 'singlethread' is unchanged, but now encoded as the
largest integer (which happens to be ~0U in bitcode).<br>
</li>
<li>New syntax "synchscope(n)" for other scopes.</li>
<li>There is no keyword for cross-thread, but it can be specified
as "synchscope(0)".</li>
</ol>
<p>This change breaks forward compatibility for the bitcode, since
the meaning of the zero/one values are now changed.<br>
</p>
<p><tt> enum SynchronizationScope {</tt><tt><br>
</tt><tt>- SingleThread = 0,</tt><tt><br>
</tt><tt>- CrossThread = 1</tt><tt><br>
</tt><tt>+ CrossThread = 0,</tt><tt><br>
</tt><tt>+ SingleThread = ~0U</tt><tt><br>
</tt><tt> };</tt><br>
</p>
<p>The change passes almost all lit tests including one new test
(see patch 0005). The failing tests are specifically checking for
forward compatibility:<br>
<br>
Failing Tests (3):<br>
LLVM :: Bitcode/cmpxchg-upgrade.ll<br>
LLVM :: Bitcode/memInstructions.3.2.ll<br>
LLVM :: Bitcode/weak-cmpxchg-upgrade.ll<br>
<br>
This breakage remains even if we reverse the order of
synchronization scopes. One simple way to preserve compatibility
is to retain 0 and 1 with their current meanings, and specify that
intermediate scopes are represented in an ordered way with numbers
greater than one. But this is pretty ugly to work with. Would
appreciate inputs on how to fix this!<br>
</p>
<p>Sameer.<br>
<br>
</p>
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