[SR-3946] Figure out the number of registers designated to hold the function result value

[ematejska]

Previous ID	SR-3946
Radar	rdar://problem/31419827
Original Reporter	@ematejska
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AffectsABI
Assignee	None
Priority	Medium

md5: 1ed01cd2ffc3ef7ce2ef8e1964b7e4f9

Issue Description:

Function signature lowering is the mapping of a function's source-language type, which includes formal parameters and results, all the way down to a physical convention, which dictates what values are stored in what registers and what values to pass on the stack.

Lowering the result value is usually done first, with a certain number of registers designated to hold the result value if it fits, otherwise the result value is passed on the stack. Ahead of ABI stability, a good heuristic is needed for the limit and is architecture specific.

[bob-wilson]

@swift-ci create

[aschwaighofer]

The swift calling convention as currently implemented limit is capped to 4 registers for integer and floating point registers on 64bit platforms and ARM and 3 registers on i386.

i386 uses 3 registers:
https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/TargetInfo.cpp#L983

x86_64, AArch64,SyztemZ, ARM use 4 registers:
https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/SwiftCallingConv.cpp#L576
https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/TargetInfo.cpp#L2117
https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/TargetInfo.cpp#L2150
https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/TargetInfo.cpp#L4806
https://github.com/apple/swift-clang/blob/stable/lib/CodeGen/TargetInfo.cpp#L5381

[bob-wilson]

We're happy with that current implementation and do not plan changes in this area for the stable ABI.