Optimize failureBreakpoint().

[grynspan]

failureBreakpoint() currently has optimizations disabled which produces some significant assembly for what's supposed to be a no-op. Enabling optimizations and marking its magic value as @exclusivity(unchecked) reduces it to a single indirect store.

This is probably not the hottest hot path in the library (since it will only be called when a test is failing) but the whole point of the function is to be a no-op, and without this change it's quite noisy and includes a whole function call into the Swift runtime (see the bl instructions below.)

arm64

Before

_$s7Testing17failureBreakpointyyF: 0000000000000660subsp, sp, #0x30 0000000000000664stp	x29, x30, [sp, #0x20] 0000000000000668addx29, sp, #0x20 000000000000066c	adrp	x8, 0 ; 0x0 0000000000000670addx0, x8, #0x0 0000000000000674addx1, sp, #0x8 0000000000000678movx2, #0x21 000000000000067cmovx3, #0x0 0000000000000680bl0x680 0000000000000684movx8, #0x1 0000000000000688adrp	x9, 0 ; 0x0 000000000000068caddx9, x9, #0x0 0000000000000690strx8, [x9] 0000000000000694addx0, sp, #0x8 0000000000000698bl0x698 000000000000069c	ldp	x29, x30, [sp, #0x20] 00000000000006a0addsp, sp, #0x30 00000000000006a4ret

After

_$s7Testing17failureBreakpointyyF: 0000000000000660adrp	x8, 0 ; 0x0 0000000000000664strxzr, [x8] 0000000000000668ret

Note this disassembly comes from the intermediate .o file generated for the relevant source file, so addresses aren't available yet. Neither function actually attempts to write to 0x0.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[grynspan]

@swift-ci test

[stmontgomery]

Nice!