…saethlin

miri weak memory emulation: put previous value into initial store buffer

Fixes rust-lang/miri#2164 by doing a read before each atomic write so that we can initialize the store buffer. The read suppresses memory access hooks and UB exceptions, to avoid otherwise influencing the program behavior. If the read fails, we store that as `None` in the store buffer, so that when an atomic read races with the first atomic write to some memory and previously the memory was uninitialized, we can report UB due to reading uninit memory.

`@cbeuw` this changes a bit the way we initialize the store buffers. Not sure if you still remember all this code, but if you could have a look to make sure this still makes sense, that would be great. :)

r? `@saethlin`

…kingjubilee

Rollup of 9 pull requests

Successful merges:

 - rust-lang#126985 (Implement `-Z embed-source` (DWARFv5 source code embedding extension))
 - rust-lang#127922 (Add unsafe to extern blocks in style guide)
 - rust-lang#128731 (simd_shuffle intrinsic: allow argument to be passed as vector)
 - rust-lang#128935 (More work on `zstd` compression)
 - rust-lang#128942 (miri weak memory emulation: put previous value into initial store buffer)
 - rust-lang#129418 (rustc: Simplify getting sysroot library directory)
 - rust-lang#129490 (Add Trusty OS as tier 3 target)
 - rust-lang#129559 (float types: document NaN bit pattern guarantees)
 - rust-lang#129642 (Bump backtrace to rust-lang/backtrace@fc37b22)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 9 pull requests

Successful merges:

 - rust-lang#126985 (Implement `-Z embed-source` (DWARFv5 source code embedding extension))
 - rust-lang#127922 (Add unsafe to extern blocks in style guide)
 - rust-lang#128731 (simd_shuffle intrinsic: allow argument to be passed as vector)
 - rust-lang#128935 (More work on `zstd` compression)
 - rust-lang#128942 (miri weak memory emulation: put previous value into initial store buffer)
 - rust-lang#129418 (rustc: Simplify getting sysroot library directory)
 - rust-lang#129490 (Add Trusty OS as tier 3 target)
 - rust-lang#129536 (Add `f16` and `f128` inline ASM support for `aarch64`)
 - rust-lang#129559 (float types: document NaN bit pattern guarantees)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#128942 - RalfJung:interpret-weak-memory, r=saethlin

miri weak memory emulation: put previous value into initial store buffer

Fixes rust-lang/miri#2164 by doing a read before each atomic write so that we can initialize the store buffer. The read suppresses memory access hooks and UB exceptions, to avoid otherwise influencing the program behavior. If the read fails, we store that as `None` in the store buffer, so that when an atomic read races with the first atomic write to some memory and previously the memory was uninitialized, we can report UB due to reading uninit memory.

``@cbeuw`` this changes a bit the way we initialize the store buffers. Not sure if you still remember all this code, but if you could have a look to make sure this still makes sense, that would be great. :)

r? ``@saethlin``