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Re-enable atomic loads and stores for all RISC-V targets
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This roughly reverts PR #66548

Atomic "CAS" are still disabled for targets without the
*“A” Standard Extension for Atomic Instructions*.
However this extension only adds instructions for operations more complex
than simple loads and stores, which are always atomic when aligned.

In the [Unprivileged Spec v. 20191213](https://riscv.org/technical/specifications/)
section 2.6 *Load and Store Instructions* of
chapter 2 *RV32I Base Integer Instruction Set* (emphasis mine):

> Even when misaligned loads and stores complete successfully,
> these accesses might run extremely slowly depending on the implementation
> (e.g., when implemented via an invisible trap). Further-more, whereas
> **naturally aligned loads and stores are guaranteed to execute atomically**,
> misaligned loads and stores might not, and hence require
> additional synchronization to ensure atomicity.

Unfortunately PR #66548 did not provide
much details on the bug that motivated it, but
#66240 and
#85736 appear related
and happen with targets that do have the A extension.
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SimonSapin committed Jun 21, 2022
1 parent abace0a commit 20bd0c3
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Showing 3 changed files with 3 additions and 3 deletions.
Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ pub fn target() -> Target {
linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
linker: Some("rust-lld".into()),
cpu: "generic-rv32".into(),
max_atomic_width: Some(0),
max_atomic_width: Some(32),
atomic_cas: false,
executables: true,
panic_strategy: PanicStrategy::Abort,
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ pub fn target() -> Target {
linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
linker: Some("rust-lld".into()),
cpu: "generic-rv32".into(),
max_atomic_width: Some(0),
max_atomic_width: Some(32),
atomic_cas: false,
features: "+m".into(),
executables: true,
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ pub fn target() -> Target {
linker_flavor: LinkerFlavor::Lld(LldFlavor::Ld),
linker: Some("rust-lld".into()),
cpu: "generic-rv32".into(),
max_atomic_width: Some(0),
max_atomic_width: Some(32),
atomic_cas: false,
features: "+m,+c".into(),
executables: true,
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