codegen: give RMW fields a slot when splitting an allocation (#62752)
Fix the recorded effects for RMW operations in alloc-opt:
`AllocOpt`'s escape analysis recorded `cmpxchg` and `atomicrmw` as pure
stores of their new-value operand, so the field they touch was left with
`hasload == false`. `splitOnStack` skips allocating a stack slot for
such a field, and then indexes the empty slot vector while rewriting the
atomic, reading whatever happened to be on the stack. With assertions
this trips `SmallVector::operator[]`; without them it produces a bogus
pointer operand and crashes further down the pass pipeline.
`addMemOp` now takes separate `isload` and `isstore` flags, and the
atomics set both, which keeps `refstore` set for the write-barrier check
in `JuliaLICM` while also marking the field as read.
Fixes #52575
Disclaimer: Claude did the debugging here, so I didn't verify the bug,
but the change seems right regardless and the test seems okay to show it
now works on this example.
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>
julia
08963f68 - codegen: give RMW fields a slot when splitting an allocation (#62752)
