Commit ef4915f
doc: fix Fast FFI argument count in ffi.md
The text said functions with more than 7 arguments fall back to the
generic call path, contradicting the preceding sentence that Fast FFI
calls support up to 8 arguments.
The real limit is architecture- and type-dependent: the hard cap is 8
public arguments (src/ffi/fast.cc), but register pressure lowers the
effective count to 7 integer/pointer arguments on AArch64 and 6 on
x86-64, while floating-point arguments can use up to 8 on both. Describe
this instead of the previous, inaccurate single threshold.
Signed-off-by: Daijiro Wachi <daijiro.wachi@gmail.com>
PR-URL: #63960
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Paolo Insogna <paolo@cowtech.it>1 parent 518309c commit ef4915f
1 file changed
Lines changed: 7 additions & 2 deletions
| Original file line number | Diff line number | Diff line change | |
|---|---|---|---|
@@ -125,8 +125,13 @@ raw pointer `bigint` values. For pointer-like parameters, `null`, `undefined`, | |||
| 125 | 125 | strings, `Buffer`, typed array, `DataView`, and `ArrayBuffer` values are | |
| 126 | 126 | converted on the JavaScript side before calling the optimized native wrapper. | |
| 127 | 127 | ||
| 128 | - Optimized Fast FFI calls support at most 8 function arguments. Functions with | ||
| 129 | - more than 7 arguments use the generic FFI call path instead. | ||
| 128 | + Optimized Fast FFI calls support at most 8 function arguments, but the exact | ||
| 129 | + limit depends on the architecture and on the argument types, because each | ||
| 130 | + argument must fit in the registers used by the platform trampoline. Integer | ||
| 131 | + and pointer arguments are limited to 7 on AArch64 and to 6 on x86-64, while | ||
| 132 | + floating-point arguments can use up to 8 on both. Functions that exceed these | ||
| 133 | + limits, including any function with more than 8 arguments, use the generic FFI | ||
| 134 | + call path instead. | ||
| 130 | 135 | ||
| 131 | 136 | ## Signature objects | |
| 132 | 137 | ||
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