[PATCH] D85710: Code generation support for lvalue bit-field conditionals.

[Richard Smith - zygoloid via Phabricator via cfe-commits]

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C++ permits use of (cond ? x.a : y.b) as an lvalue even when a and/or b is a
bit-field. We previously failed to generate IR for such constructs.

Handle these cases with a new form of LValue representing a choice between two
other LValues. Whenever we want to generate a primitive operation on a
conditional lvalue, generate a branch to perform the operation.

This would be sufficient to also support lvalue conditionals between other
kinds of non-simple lvalue, such as a conditional between the imaginary
component of a _Complex T and a vector element, but Sema rejects all the other
combinations for the moment.

One case that's not yet supported is atomic compare-exchange operations on
bitfield conditional lvalues. These only arise through `#pragma omp atomic`
constructs, and would probably not be too hard to support, but we need to teach
the OpenMP code to cope with its subexpression being emitted multiple times.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D85710

Files:
  clang/lib/CodeGen/CGAtomic.cpp
  clang/lib/CodeGen/CGExpr.cpp
  clang/lib/CodeGen/CGStmtOpenMP.cpp
  clang/lib/CodeGen/CGValue.h
  clang/lib/CodeGen/CodeGenFunction.h
  clang/test/CodeGenCXX/conditional-expr-lvalue.cpp

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