Poor quality stencils produced with Homebrew clang version 22.1.2 on Apple Silicon

While working on https://github.com/python/cpython/pull/148217, I found that the _fixup_constants optimizer in Tools/jit/_optimizers.py fails to convert adrp+ldr to movz when the two instructions are split by a label.

Example: _LOAD_FAST_r01

// Before (8 instructions):
adrp    x8, __JIT_OPARG_16@GOTPAGE
ldr     x8, [x8, __JIT_OPARG_16@GOTPAGEOFF]
add     x8, x21, w8, uxth #3
ldr     x24, [x8, #0x50]
tbnz    w24, #0x0, ...
ldr     w8, [x24]
add     w8, w8, #0x1
str     w8, [x24]

// After (7 instructions):
movz    x8, #0x0
add     x8, x21, w8, uxth #3
ldr     x24, [x8, #0x50]
tbnz    w24, #0x0, ...
ldr     w8, [x24]
add     w8, w8, #0x1
str     w8, [x24]

Example: _LOAD_FAST_BORROW_r01

// Before (5 instructions):
adrp    x8, __JIT_OPARG_16@GOTPAGE
ldr     x8, [x8, __JIT_OPARG_16@GOTPAGEOFF]
add     x8, x21, w8, uxth #3
ldr     x8, [x8, #0x50]
orr     x24, x8, #0x1

// After (4 instructions):
movz    x8, #0x0
add     x8, x21, w8, uxth #3
ldr     x8, [x8, #0x50]
orr     x24, x8, #0x1

Linked PRs

• gh-148436
• gh-157040