[lld] [lld][MachO] Add N_COLD_FUNC support (PR #183909)

[Ellis Hoag via llvm-commits]

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+# REQUIRES: aarch64
+
+## Test that when two weak definitions of the same symbol exist and either one
+## is marked N_COLD_FUNC, the merged symbol is treated as cold. This matches the
+## behavior of both ld-prime and ld64.
+
+# RUN: rm -rf %t; split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/weak-cold.s -o %t/weak-cold.o
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/weak-noncold.s -o %t/weak-noncold.o
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/weak-main.s -o %t/weak-main.o
+
+## Link order: non-cold first, cold second. The non-cold def wins but cold |= true.
+# RUN: %lld -arch arm64 -lSystem %t/weak-noncold.o %t/weak-cold.o %t/weak-main.o -o %t/weak-nc-c.exe
+# RUN: llvm-objdump -d %t/weak-nc-c.exe | FileCheck %s
+
+## Link order: cold first, non-cold second. The cold def wins and cold |= false.
+# RUN: %lld -arch arm64 -lSystem %t/weak-cold.o %t/weak-noncold.o %t/weak-main.o -o %t/weak-c-nc.exe
+# RUN: llvm-objdump -d %t/weak-c-nc.exe | FileCheck %s
+
+## In both link orders, _weakfn ends up cold and is placed after _main.
+# CHECK: <_main>:
+# CHECK: <_weakfn>:
+
+#--- weak-cold.s
+.subsections_via_symbols
+.text
+.globl _weakfn
+.weak_definition _weakfn
+.p2align 2
+.desc _weakfn, 0x400
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ellishg wrote:

I think a comment would be nice. It is clear from the name that this is cold, but I would still like to understand the mechanics of how that is specified.

https://github.com/llvm/llvm-project/pull/183909