perf: integer specialization for loop-stable int32 locals — skip scvtf + NaN-box round-trip #48

Context

Companion to #47. honest_bench image_conv profile of Perry v0.5.30 shows two back-to-back bl calls per byte read in the inner blur loop: one for the buffer read (#47), one for a NaN-box wrapper. This issue is the second call.

At 0x100c in the profiled binary:

scvtf d0, w0              ; i32 result from buffer_get → f64
bl    0x10001edac         ; NaN-box wrapper — full frame, tag-table setup, check, return

The wrapper function starts:

sub   sp, sp, #0x80
stp   x26, x25, [sp, #0x30]
...                       ; 5× stp register spills
mov   x20, #0x1;  movk  x20, #0x7ffc, lsl #48   ; TAG_UNDEFINED
mov   x21, #0x2;  movk  x21, #0x7ffc, lsl #48   ; TAG_NULL
mov   x22, #-0x3; movk  x22, #0x8003, lsl #48
mov   x23, #0x7fff000000000000                  ; STRING_TAG
mov   w24, #0x7ffe0000                          ; INT32_TAG
mov   x25, #0x7ffd000000000000                  ; POINTER_TAG
fmov  x19, d0
cmp   x19, x20                                   ; dispatch by tag
...

— i.e. it's a generic "coerce any JSValue to double" that sets up the full tag table to handle every possible incoming tag. When called right after scvtf it's doing pointless work: the incoming value is known to be a freshly-converted double from a known-int32 source.

Proposal

Type inference already tracks stability for some locals. Extend it (or add a loop-local pass) to mark a number as int32-stable when:

• its initializer is an integer literal, |0, >>> 0, idx * k, +/-/* of other int32-stable locals, or a Buffer[idx] read (after perf: inline Buffer/Uint8Array bracket-read when statically typed (9 instr + bl → 1 instr) #47),
• all assignments in scope are also int32-stable, and
• no observable floating-point consumer (e.g. it never flows into Math.sqrt, /, Math.log, etc.).

For such locals, the codegen emits i32 throughout its live range: loads/stores stay as ldr w/str w, arithmetic stays as mul w/add w, and NaN-boxing happens only at the escape boundary (store to heap, argument pass to a generic callee, print, etc.).

Expected impact

Stacks on top of #47. With #47 already in place, the blur loop would be:

ldrb w0, [x_src, w_idx, uxtw]     ; from #47
mul  w0, w0, w_k                  ; keep i32
add  w_acc, w_acc, w0

— instead of ldrb → scvtf → bl nan_box → fmadd. Removes the second bl entirely (the profile showed it taking more samples than the first one — ~30+ instrs vs js_buffer_get's 9).

On the image_conv workload with #47 already applied, projected additional ~2× speedup. Combined with #47's autovectorization unblock, lands Perry ahead of Rust on tight compute loops.

Scope

• Pixel-heavy code: image convolution, resize, colorspace conversion, compositing.
• Hash loops: FNV, MurmurHash, xxhash (the honest_bench/…/perry/image_conv.ts uses a hand-rolled imul32 that would benefit).
• Byte parsers: base64, UTF-8 validators, varint.

Counter-shape to be careful of

Locals that look int32 but occasionally hold NaN or Infinity (e.g. const n = parseInt(userInput) where userInput might be "abc") must not be specialized. The analysis has to trace the flow and bail on any edge where a non-int32 can enter. The compile-time type (number in TS) is not enough; actual use has to be checked.