jmp_match: account for self-shrink on forward jcc/jmp rel8 check#239
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jmp_match: account for self-shrink on forward jcc/jmp rel8 check#239bboe wants to merge 1 commit into
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The borderline rel8 fit test compared the current-layout short displacement (target - offset - short_size). For a forward jump that the previous pass emitted as NEAR, shrinking to SHORT moves the target near_size - short_size bytes closer, so the post-shrink rel8 is that many bytes smaller. NASM was keeping forward near jcc/jmp instances that a self-shrink-aware iteration emits as short (a 4-byte shrink for jcc32, 3-byte for jmp32). Track per-jump previous-pass encoding in a static side table, swapped at pass boundaries via _passn. In the borderline branch, when the current-layout check rejects AND the previous pass was NEAR AND the jump is forward, re-check against the post-shrink displacement. The previous-pass guard prevents over-shrinking already-short jumps whose displacement grew past 127; the forward guard skips backward jumps, whose target doesn't move when the jump shrinks. Add travis/test/jccshrink as a regression test: stock NASM emits 605 B with a 5-byte near jmp at 0x1d9; patched emits 602 B with eb 7f (rel8 = 127). make travis passes (301/301). Byte-identical on optimization.asm, jmp64.asm, riprel.asm, a32offs.asm. Signed-off-by: Bryce Boe <bbzbryce@gmail.com>
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Forward jcc/jmp self-shrink case in
jmp_match()'s borderline rel8 fit check was rejecting valid shrinks because it compared against the current-layout displacement instead of the post-shrink one. Regression test:travis/test/jccshrink.