What does cmpq $0x7, %reg + ja pattern check?

It rejects anything outside 0-7 with one unsigned comparison — negatives wrap to huge unsigned values, so ja catches both "too big" and "negative" at once.

cmpl $0x7, %edi
ja   .L_default

This explodes/goes to default if %edi > 7 unsigned. Since negative numbers are huge unsigned values, this simultaneously catches:

• Values > 7 (too large)
• Negative values (wrap to large unsigned)

Where it appears: Almost always as a switch statement bounds check before a jump table.

General pattern: cmp $N, %reg + ja label ensures 0 <= reg <= N in a single check.

Go deeper:

• Wikipedia: Branch table — the unsigned bounds check that guards a dense switch's jump table
• Intel JMP reference — the indirect jump this bounds check typically protects