Merge #70

70: aarch64: Use CASP instead of LDXP/STXP for store/RMW if available r=taiki-e a=taiki-e

Follow-up #20

In #20, we postponed this (#20 (comment)), but LLVM 16+ adopted this (llvm/llvm-project@10d34f5), so we'll follow LLVM's behavior.

Co-authored-by: Taiki Endo <te316e89@gmail.com>

.github/bors.toml

@@ -4,4 +4,4 @@ status = [
     "test (aarch64-apple-darwin)",
     "valgrind (aarch64-unknown-linux-gnu)",
 ]
-timeout_sec = 7200
+timeout_sec = 10800

.github/workflows/ci.yml

@@ -210,7 +210,7 @@ jobs:
           RUSTDOCFLAGS: ${{ env.RUSTDOCFLAGS }} -C target-feature=+lse ${{ env.RANDOMIZE_LAYOUT }}
           RUSTFLAGS: ${{ env.RUSTFLAGS }} -C target-feature=+lse ${{ env.RANDOMIZE_LAYOUT }}
         if: startsWith(matrix.target, 'aarch64') && startsWith(matrix.rust, 'nightly') && !(contains(matrix.target, '-musl') || contains(matrix.target, '-android'))
-      # TODO: it seems qemu-user has not yet properly implemented FEAT_LSE2: https://github.com/taiki-e/portable-atomic/pull/11#issuecomment-1114044327
+      # TODO: As of QEMU 7.2, QEMU has not yet implemented FEAT_LSE2: https://linaro.atlassian.net/browse/QEMU-300
 
       # pwr7
       # powerpc64- (big-endian) is skipped because it is pre-pwr8 by default

build.rs

@@ -242,6 +242,8 @@ fn main() {
                 }
             }
             // lqarx and stqcx.
+            // Note: As of rustc 1.67, target_feature "quadword-atomics" is not available on rustc side:
+            // https://github.com/rust-lang/rust/blob/1.67.0/compiler/rustc_codegen_ssa/src/target_features.rs#L215
             target_feature_if("quadword-atomics", has_pwr8_features, &version, None, false);
         }
         _ => {}

src/imp/atomic128/README.md

@@ -2,11 +2,13 @@
 
 The table of targets that support 128-bit atomics and the instructions used:
 
-| target_arch | load | store | CAS | note |
-| ----------- | ----- | ----- | ---- | ---- |
-| x86_64 | cmpxchg16b or vmovdqa | cmpxchg16b or vmovdqa | cmpxchg16b | cmpxchg16b target feature required. vmovdqa requires Intel or AMD CPU with AVX. <br> Both compile-time and run-time detection are supported for cmpxchg16b. vmovdqa is currently run-time detection only.  <br> Requires rustc 1.59+ when cmpxchg16b target feature is enabled at compile-time, otherwise requires nightly |
-| aarch64 | ldxp/stxp or casp or ldp | ldxp/stxp or stp | ldxp/stxp or casp | casp requires lse target feature, ldp/stp requires lse2 target feature. <br> Both compile-time and run-time detection are supported for lse. lse2 is currently compile-time detection only.  <br> Requires rustc 1.59+ |
-| powerpc64 | lq | stq | lqarx/stqcx. | Little endian or target CPU pwr8+. <br> Requires nightly |
-| s390x | lpq | stpq | cdsg | Requires nightly |
+| target_arch | load  | store | CAS  | RMW  | note |
+| ----------- | ----- | ----- | ---- | ---- | ---- |
+| x86_64 | cmpxchg16b or vmovdqa | cmpxchg16b or vmovdqa | cmpxchg16b | cmpxchg16b | cmpxchg16b target feature required. vmovdqa requires Intel or AMD CPU with AVX. <br> Both compile-time and run-time detection are supported for cmpxchg16b. vmovdqa is currently run-time detection only.  <br> Requires rustc 1.59+ when cmpxchg16b target feature is enabled at compile-time, otherwise requires nightly |
+| aarch64 | ldxp/stxp or casp or ldp | ldxp/stxp or casp or stp | ldxp/stxp or casp | ldxp/stxp or casp | casp requires lse target feature, ldp/stp requires lse2 target feature. <br> Both compile-time and run-time detection are supported for lse. lse2 is currently compile-time detection only.  <br> Requires rustc 1.59+ |
+| powerpc64 | lq | stq | lqarx/stqcx. | lqarx/stqcx. | Little endian or target CPU pwr8+. <br> Requires nightly |
+| s390x | lpq | stpq | cdsg | cdsg | Requires nightly |
 
 Run-time detections are enabled by default and can be disabled with `--cfg portable_atomic_no_outline_atomics`.
+
+See [aarch64.rs](aarch64.rs) module-level comments for more details on the instructions used on aarch64.