[5.5] async function pointer support for Windows #40147
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With PE/COFF, one cannot reference a data symbol directly across the
binary module boundary. Instead, the reference must be indirected
through the Import Address Table (IAT) to allow for position
independence.
When generating a reference to a AsyncFunctionPointer ({i8*, i32}), we
tag the pointer as being indirected by tagging bit 1 (with the
assumption that native alignment will ensure 4/8 byte alignment, freeing
the bottom 2 bits at least for bit-packing). We tweak the
v-table/witness table emission such that all references to the
AsyncFunctionPointer are replaced with the linker synthetic import
symbol with the bit packing:
~~~
.quad __imp_$s1L1CC1yyYaKFTu+1
~~~
rather than
~~~
.quad $s1L1CC1yyYaKFTu
~~~
Upon access of the async function pointer reference, we open-code the
check for the following:
~~~
pointer = (pointer & 1) ? *(void **)(pointer & ~1) : pointer;
~~~
Thanks to @DougGregor for the discussion and the suggestion for the
pointer tagging. Thanks to @aschwaighofer for pointers to the code that
I had missed. Also, thanks to @SeanROlszewski for the original code
sample that led to the reduced test case.
Fixes: SR-15399
(cherry picked from commit 68bc33f)
Tweak test for macOS (cherry picked from commit fee023c)
|
@swift-ci please test |
Make the test less restrictive. The value of the select is not referenced, no need to match it.
|
@swift-ci please test |
|
@swift-ci please test Windows platform |
|
I'll try to do some more testing over the weekend. |
tkremenek
approved these changes
Nov 15, 2021
|
@compnerd is this OK to merge? |
|
@tkremenek thanks, yes, this should be good to merge. |
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I'm nominating this for Swift 5.5 on behalf of my team (XCTest) and the original PR author (@compnerd), since it supports Swift async test support. Our XCTest release manager @bcroom is out on vacation currently, but he approved this via Slack.
Explanation: This fixes an issue leading to linker failures with open async functions on Windows. For detailed summary of the problem, see the original PR (IRGen: initial pass to support
asyncinheritance on Windows #39998). The motivation for bringing it back to 5.5 is that this is one PR of several which support async XCTests, since this problem was identified as a blocker to landing the Corelibs XCTest changes for Windows.Scope: Limited. Affects code which has adopted async on Windows, and uses subclassing.
SR Issue: SR-15399
Risk: Low, only affecting PE/COFF (Windows), relatively new code which has adopted async, under narrow circumstances.
Testing: New unit tests are included, it passes existing tests and smoke tests as well. Change has been on main for 2 weeks without issue. Also resolves an existing XFAIL’ed test on Windows.
Reviewer: @aschwaighofer