arm64: Emit tbnz/tbz for sign-bit branch tests in branchTest32/branchTestPtr

👁 Image	Andrew Gaul Assignee
	Description • 2 months ago

branchTest32 and branchTestPtr in MacroAssembler-arm64-inl.h already recognize the test reg, reg idiom (where lhs == rhs) and emit cbz/cbnz for the Zero/NonZero conditions. However, the Signed/NotSigned conditions fall through to the generic path, which emits two instructions:

tst w0, w0
b.mi label

This patch extends the existing pattern to also handle Signed and NotSigned, emitting a single tbnz/tbz on the sign bit instead:

tbnz w0, #31, label (32-bit Signed)
tbz w0, #31, label (32-bit NotSigned)
tbnz x0, #63, label (64-bit Signed)
tbz x0, #63, label (64-bit NotSigned)

This saves one instruction per call site, avoids clobbering NZCV flags, and matches what LLVM already emits for equivalent sign-bit tests in AOT-compiled code (where sign-bit tbz/tbnz account for 1-4% of all instructions in typical arm64 binaries).

Call sites that benefit include visitModPowTwoI, visitModPowTwoI64, bailoutTest32(Signed, ...), and any other path through branchTest{32,Ptr}(Signed/NotSigned, r, r, ...).

Range safety: tbnz/tbz have a ±32KB offset (14-bit immediate), tighter than b.cc's ±1MB. Mozilla's vixl fork handles this via registerBranchDeadline with automatic veneer insertion, so arbitrarily distant labels degrade gracefully to a 2-instruction trampoline — no worse than the original code.

Verified with IONFLAGS=codegen that Ion emits tbnz/tbz for the targeted patterns. ./mach jit-test passes.

👁 Image	Andrew Gaul Assignee
	Comment 1 • 2 months ago

Extend branchTest32/branchTestPtr to recognize the (Signed/NotSigned, r, r)
pattern and emit a single tbnz/tbz on bit 31/63 instead of tst + b.mi/b.pl.

👁 Image	Phabricator Automation
	Updated • 2 months ago

👁 Image	Bryan Thrall [:bthrall]
	Updated • 1 month ago

👁 Image	André Bargull [:anba]
	Updated • 1 month ago

👁 Image	Pulsebot
	Comment 2 • 1 month ago

Pushed by andre.bargull@gmail.com: https://github.com/mozilla-firefox/firefox/commit/d5645d361f78 https://hg.mozilla.org/integration/autoland/rev/ff5f0f6305cd arm64: Emit tbnz/tbz for sign-bit branch tests. r=jandem,anba

👁 Image	Cosmin Sabou [:CosminS]
	Comment 3 • 1 month ago
bugherder

https://hg.mozilla.org/mozilla-central/rev/ff5f0f6305cd

👁 Image	Donal Meehan [:dmeehan]
	Updated • 1 month ago

👁 Image	Mihai Boldan, Desktop QA [:mboldan]
	Updated • 26 days ago