tree 1ffb77d71b2e83ab6625126da2ac29c71753316a
parent b2d6212d1a051d750fbbc6e71607ad76153e04f0
author Tom Stellard <thomas.stellard@amd.com> 1481054973 +0000
committer Tom Stellard <thomas.stellard@amd.com> 1481054973 +0000

Merging r288433:

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r288433 | oranevskyy | 2016-12-01 14:58:35 -0800 (Thu, 01 Dec 2016) | 24 lines

[ARM] Fix for 64-bit CAS expansion on ARM32 with -O0

Summary:
This patch fixes comparison of 64-bit atomic with its expected value in CMP_SWAP_64 expansion.

Currently, the low words are compared with CMP, while the high words are compared with SBC. SBC expects the carry flag to be set if CMP detects a difference. CMP might leave the carry unset for unequal arguments though if the first one is >= than the second. This might cause the comparison logic to detect false equality.

Example of the broken C++ code:
```
std::atomic<long long> at(2);

long long ll = 1;
std::atomic_compare_exchange_strong(&at, &ll, 3);
```
Even though the atomic `at` and the expected value `ll` are not equal and `atomic_compare_exchange_strong` returns `false`, `at` is changed to 3.

The patch replaces SBC with CMPEQ.

Reviewers: t.p.northover

Subscribers: aemerson, rengolin, llvm-commits, asl

Differential Revision: https://reviews.llvm.org/D27315

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_39@288847 91177308-0d34-0410-b5e6-96231b3b80d8