[5.3] Fix a miscompile of global variables #34495
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) GlobalOpt works mostly on trivial values (there are special cases for ObjectInst and ValueToBridgeObjectInst). optimizeGlobalAccess is explicitly turned off for non-trivial values. optimizeInitializer calls SILGlobalVariable::isValidStaticInitializerInst which limits it to mostly trivial values except for special cases for ObjectInst and ValueToBridgeObjectInst. This changes adds GlobalOpt tests for ossa and enables GlobalOpt on ossa
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@swift-ci test |
meg-gupta
approved these changes
Oct 29, 2020
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@eeckstein this is fine for 5.3, but I would not call the bug "fixed" until |
airspeedswift
approved these changes
Oct 30, 2020
AnthonyLatsis
added
swift 5.3
🍒 release cherry pick
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This is a cherry pick of #32621
What we didn't know in the original PR is that it's actually fixing a bad miscompile in GlobalOpt, which was introduced in swift 5.1 by #22967.
Radar: rdar://problem/70399912, Jira: https://bugs.swift.org/browse/SR-13748
Explanation: In case a function is in ownership-SSA form, the global-variable-optimization skipped checking for accesses to global variables. This can lead to constant propagating a global variable (e.g. if it's an Int), even if there are multiple writes with different values to that variable. The weird thing is that we fixed the bug on the main branch since a while - without knowing that the change is actually a bug fix.
Scope: The bug was introduced in Swift 5.1. It can result in miscompiles if an internal or private global (or static) variable is set to different constant values at multiple locations in the source code.
Risk: Low. The bug was fixed on main since a while and the compiler tested since then. Also, the code change is trivial.
Reviewer: @meg-gupta