Initial RISC-V AAL

src/aal/aal.h

@@ -32,6 +32,10 @@
 #  define PLATFORM_IS_SPARC
 #endif
 
+#if defined(__riscv)
+#  define PLATFORM_IS_RISCV
+#endif
+
 namespace snmalloc
 {
   /**
@@ -218,6 +222,8 @@ namespace snmalloc
 #  include "aal_powerpc.h"
 #elif defined(PLATFORM_IS_SPARC)
 #  include "aal_sparc.h"
+#elif defined(PLATFORM_IS_RISCV)
+#  include "aal_riscv.h"
 #endif
 
 namespace snmalloc

src/aal/aal_consts.h

@@ -33,5 +33,6 @@ namespace snmalloc
     X86,
     X86_SGX,
     Sparc,
+    RISCV
   };
 } // namespace snmalloc

src/aal/aal_riscv.h

@@ -0,0 +1,54 @@
+#pragma once
+
+#if __riscv_xlen == 64
+#  define SNMALLOC_VA_BITS_64
+#elif __riscv_xlen == 32
+#  define SNMALLOC_VA_BITS_32
+#endif
+
+namespace snmalloc
+{
+  /**
+   * RISC-V architecture layer, phrased as generically as possible.  Specific
+   * implementations may need to adjust some of these.
+   */
+  class AAL_RISCV
+  {
+  public:
+    static constexpr uint64_t aal_features = IntegerPointers;
+
+    static constexpr size_t smallest_page_size = 0x1000;
+
+    static constexpr AalName aal_name = RISCV;
+
+    static void inline pause()
+    {
+      /*
+       * The "Zihintpause" extension claims to be the right thing to do here,
+       * and it is expected to be used in analogous places, e.g., Linux's
+       * cpu_relax(), but...
+       *
+       *  its specification is somewhat unusual, in that it talks about the rate
+       *  at which a HART's instructions retire rather than the rate at which
+       *  they are dispatched (Intel's PAUSE instruction explicitly promises
+       *  that it "de-pipelines" the spin-wait loop, for example) or anything
+       *  about memory semantics (Intel's PAUSE docs talk about a possible
+       *  memory order violation and pipeline flush upon loop exit).
+       *
+       *  we don't yet have examples of what implementations have done.
+       *
+       *  it's not yet understood by C frontends or assembler, meaning we'd have
+       *  to spell it out by hand, as
+       *      __asm__ volatile(".byte 0xF; .byte 0x0; .byte 0x0; .byte 0x1");
+       *
+       * All told, we just leave this function empty for the moment.  The good
+       * news is that, if and when we do add a PAUSE, the instruction is encoded
+       * by stealing some dead space of the FENCE instruction and so should be
+       * available everywhere even if it doesn't do anything on a particular
+       * microarchitecture.
+       */
+    }
+  };
+
+  using AAL_Arch = AAL_RISCV;
+}