[DO NOT MERGE] RISC-V: OS-independent implication logic

RFC: where to put `imply_features`?

This commit adds the OS-independent extension implication logic for RISC-V.
It implements:

1.  Regular implication (A → B)
    a.  "the extension A implies the extension B"
    b.  "the extension A requires the extension B"
    c.  "the extension A depends on the extension B"
2.  Extension group or shorthand (A == B1 & B2...)
    a.  "the extension A is shorthand for other extensions: B1, B2..."
    b.  "the extension A comprises instructions provided by B1, B2..."
    This is implemented as (A → B1 & B2... + B1 & B2... → A)
    where the former is a regular implication as required by specifications
    and the latter is a "reverse" implication to improve usability.

and prepares for:

3.  Implication with multiple requirements (A1 & A2... → B)
    a.  "A1 + A2 implies B"
    b.  (implicitly used to implement reverse implication of case 2)

Although it uses macros and iterators, good optimizers turn the series of
implications into fast bit-manipulation operations.

In the case 2 (extension group or shorthand; where a superset extension
is just a collection of other subextensions and provides no features by
a superset itself), specifications do specify that an extension group
implies its members but not vice versa.  However, implying an extension
group from its members would improve usability on the feature detection
(especially when the feature provider does not provide existence of such
extension group but provides existence of its members).

Similar "reverse implication" on RISC-V is implemented on LLVM.

Case 3 is implicitly used to implement reverse implication of case 2 but
there's another use case: implication with multiple requirements like
"Zcf" and "Zcd" extensions (not yet implemented in this crate for now).

To handle extension groups perfectly, we need to loop implication several
times (until they converge; normally 2 times and up to 4 times when we add
most of `riscv_hwprobe`-based features).
To enable that loop, `cache::Initializer` is modified to derive `Eq`.

Author: a4lg

crates/std_detect/src/detect/cache.rs

@@ -31,7 +31,7 @@ const CACHE_CAPACITY: u32 = 93;
 /// This type is used to initialize the cache
 // The derived `Default` implementation will initialize the field to zero,
 // which is what we want.
-#[derive(Copy, Clone, Default)]
+#[derive(Copy, Clone, Default, PartialEq, Eq)]
 pub(crate) struct Initializer(u128);
 
 // NOTE: the `debug_assert!` would catch that we do not add more Features than

crates/std_detect/src/detect/os/linux/riscv.rs

@@ -3,6 +3,75 @@
 use super::auxvec;
 use crate::detect::{Feature, bit, cache};
 
+/// Imply features by the given set of enabled features.
+///
+/// Note that it does not perform any consistency checks including existence of
+/// conflicting extensions and/or complicated requirements.  Eliminating such
+/// inconsistencies is the responsibility of the feature detection logic and
+/// its provider(s).
+pub(crate) fn imply_features(mut value: cache::Initializer) -> cache::Initializer {
+    loop {
+        // Check convergence of feature flags later.
+        let prev = value;
+
+        // Expect that the optimizer turns repeated operations into
+        // a fewer number of bit-manipulation operations.
+        macro_rules! imply {
+            // Regular implication:
+            // A1 => (B1[, B2...]), A2 => (B1[, B2...]) and so on.
+            ($($from: ident)|+ => $($to: ident)&+) => {
+                if [$(Feature::$from as u32),+].iter().any(|&x| value.test(x)) {
+                    $(
+                        value.set(Feature::$to as u32);
+                    )+
+                }
+            };
+            // Implication with multiple requirements:
+            // A1 && A2 ... => (B1[, B2...]).
+            ($($from: ident)&+ => $($to: ident)&+) => {
+                if [$(Feature::$from as u32),+].iter().all(|&x| value.test(x)) {
+                    $(
+                        value.set(Feature::$to as u32);
+                    )+
+                }
+            };
+        }
+        macro_rules! group {
+            ($group: ident == $($member: ident)&+) => {
+                // Forward implication as defined in the specifications.
+                imply!($group => $($member)&+);
+                // Reverse implication to "group extension" from its members.
+                // This is not a part of specifications but convenient for
+                // feature detection and implemented in e.g. LLVM.
+                imply!($($member)&+ => $group);
+            };
+        }
+
+        group!(zkn == zbkb & zbkc & zbkx & zkne & zknd & zknh);
+        group!(zks == zbkb & zbkc & zbkx & zksed & zksh);
+        group!(zk == zkn & zkr & zkt);
+
+        group!(a == zalrsc & zaamo);
+
+        group!(b == zba & zbb & zbs);
+
+        imply!(zhinx => zhinxmin);
+        imply!(zdinx | zhinxmin => zfinx);
+
+        imply!(zfh => zfhmin);
+        imply!(q => d);
+        imply!(d | zfhmin => f);
+
+        imply!(zicntr | zihpm | zkr | f | zfinx => zicsr);
+        imply!(s | h => zicsr);
+
+        // Loop until the feature flags converge.
+        if prev == value {
+            return value;
+        }
+    }
+}
+
 /// Read list of supported features from the auxiliary vector.
 pub(crate) fn detect_features() -> cache::Initializer {
     let mut value = cache::Initializer::default();
@@ -12,22 +81,16 @@ pub(crate) fn detect_features() -> cache::Initializer {
         }
     };
 
-    // Use auxiliary vector to enable single-letter ISA extensions and Zicsr.
+    // Use auxiliary vector to enable single-letter ISA extensions.
     // The values are part of the platform-specific [asm/hwcap.h][hwcap]
     //
     // [hwcap]: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/riscv/include/uapi/asm/hwcap.h?h=v6.14
     let auxv = auxvec::auxv().expect("read auxvec"); // should not fail on RISC-V platform
     #[allow(clippy::eq_op)]
-    let has_a = bit::test(auxv.hwcap, (b'a' - b'a').into());
-    enable_feature(Feature::a, has_a);
-    enable_feature(Feature::zalrsc, has_a);
-    enable_feature(Feature::zaamo, has_a);
+    enable_feature(Feature::a, bit::test(auxv.hwcap, (b'a' - b'a').into()));
     enable_feature(Feature::c, bit::test(auxv.hwcap, (b'c' - b'a').into()));
-    let has_d = bit::test(auxv.hwcap, (b'd' - b'a').into());
-    let has_f = bit::test(auxv.hwcap, (b'f' - b'a').into());
-    enable_feature(Feature::d, has_d);
-    enable_feature(Feature::f, has_d | has_f);
-    enable_feature(Feature::zicsr, has_d | has_f);
+    enable_feature(Feature::d, bit::test(auxv.hwcap, (b'd' - b'a').into()));
+    enable_feature(Feature::f, bit::test(auxv.hwcap, (b'f' - b'a').into()));
     enable_feature(Feature::h, bit::test(auxv.hwcap, (b'h' - b'a').into()));
     enable_feature(Feature::m, bit::test(auxv.hwcap, (b'm' - b'a').into()));
 
@@ -48,5 +111,5 @@ pub(crate) fn detect_features() -> cache::Initializer {
     // to detect when Rust is used to write Linux kernel modules.
     // These should be more than Auxvec way to detect supervisor features.
 
-    value
+    imply_features(value)
 }