[llvm-bugs] [Bug 45720] New: Attribute "cold" not permitted on Blocks language extension

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45720

            Bug ID: 45720
           Summary: Attribute "cold" not permitted on Blocks language
                    extension
           Product: clang
           Version: 10.0
          Hardware: PC
                OS: All
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: cem at FreeBSD.org
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

It seems like block code is function-like enough that it makes sense to be able
to annotate the objects with __attribute__((cold)) (or hot).  Currently this is
rejected by Basic/Attr.td due to Cold/Hot SubjectLists containing only
"Function."  And I'm sure some support would be needed internally to actually
map hot/cold attributes to meaningful behavior internally.

My observation is that:

1. Blocks are useful for generating known-cold code via macros, such as in
assert or (conditional) debug-level log constructs.

2. Clang attempts to inline all blocks.

3. Even when blocks are not inlined, they are laid out adjacent to their parent
function in .text.

The desired behavior of cold-attribute Blocks is basically that the cold
attribute is passed down to the generated LLVM function corresponding to Block
invocation.  This affects NeverInline (2) and perhaps code placement (3).

As a giant kludge for (3), it seems to work to use -ffunction-sections to put
non-inlined blocks in their own sections and carefully use a linker script to
place them far away from other code.  I don't have any good workarounds for
(2).

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