[lld] [lld][MachO] Avoid quadratic iteration over already-folded symbols during ICF (PR #213339)

[Peter Rong via llvm-commits]

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+# REQUIRES: x86
+# RUN: rm -rf %t*
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-darwin %s -o %t.o
+# RUN: %lld -lSystem --icf=all -o %t %t.o
+# RUN: llvm-objdump --macho --section-headers %t | FileCheck %s --check-prefix=SECT
+# RUN: llvm-objdump --macho --syms %t | FileCheck %s --check-prefix=SYMS
+
+## Every body folds into one, so __text holds a single 6-byte body plus _main
+## rather than the 3 MiB (0x30000d) the unfolded copies occupy.
+# SECT:      Idx Name          Size
+# SECT-NEXT: 0 __text        00000009
+
+## The sentinels bracket the group, so checking that the first and last members
+## of the class share an address covers the whole run of folds.
+# SYMS-DAG: [[#%.16x,F:]] g     F __TEXT,__text _f_first
+# SYMS-DAG: [[#F]]        g     F __TEXT,__text _f_last
+
+.subsections_via_symbols
+.text
+.p2align 2
+
+.globl _f_first
+_f_first:; movl $7, %eax; ret
+
+## Unlike icf-scale.s, every body generated here is identical, so all 512 Ki
+## land in one equivalence class. \+ iterates from 0 to n-1. $$7 is an escape
+## for a literal $7: .rept expands its body as a macro with no parameters, and
+## in one of those the Darwin assembler reads $7 as positional argument 7 and
+## drops it.
+.rept 524288
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DataCorrupted wrote:

> test extremely slow and memory intensive

I believe both grow linearly. Using 512K the test can finish within a second.

I'm setting to 64K instead, empirically 32K can still finish within a sec even with quadratic loop.  

https://github.com/llvm/llvm-project/pull/213339