Re: [PATCH v2] libstdc++: Add support for POWER9 DARN instruction to std::random_device

[Jonathan Wakely <jwakely@redhat.com>]

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On Thu, 4 Nov 2021 at 20:44, Bill Schmidt wrote:

> For posterity:  This was discussed briefly on IRC, and Segher approved
> with some
> simplifications and a request to implement a fail/retry check.
>
>
Here's what I have now. No more assembler check in configure, and it uses
the 64-bit __builtin_darn() and truncates it to 32-bit, or retries (up to
100 times) if it fails.

I'm doing some more testing now.

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commit 77816264712923481db37a29d1638faa1d99aadc
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Oct 20 09:25:24 2021

    libstdc++: Add support for POWER9 DARN instruction to std::random_device
    
    The ISA-3.0 instruction set includes DARN ("deliver a random number")
    which can be used similar to the existing support for RDRAND and RDSEED.
    
    libstdc++-v3/ChangeLog:
    
            * src/c++11/random.cc [__powerpc__] (USE_DARN): Define.
            (__ppc_darn): New function to use POWER9 DARN instruction.
            (Which): Add 'darn' enumerator.
            (which_source): Check for __ppc_darn.
            (random_device::_M_init): Support "darn" and "hw" tokens.
            (random_device::_M_getentropy): Add darn to switch.
            * testsuite/26_numerics/random/random_device/cons/token.cc:
            Check "darn" token.
            * testsuite/26_numerics/random/random_device/entropy.cc:
            Likewise.

diff --git a/libstdc++-v3/src/c++11/random.cc b/libstdc++-v3/src/c++11/random.cc
index 4b64bde00ea..b0d88374d59 100644
--- a/libstdc++-v3/src/c++11/random.cc
+++ b/libstdc++-v3/src/c++11/random.cc
@@ -37,6 +37,8 @@
 # ifdef _GLIBCXX_X86_RDSEED
 #  define USE_RDSEED 1
 # endif
+#elif defined __powerpc__ && defined __BUILTIN_CPU_SUPPORTS__
+# define USE_DARN 1
 #endif
 
 #include <cerrno>
@@ -69,7 +71,7 @@
 #if defined _GLIBCXX_USE_CRT_RAND_S || defined _GLIBCXX_USE_DEV_RANDOM
 // The OS provides a source of randomness we can use.
 # pragma GCC poison _M_mt
-#elif defined USE_RDRAND || defined USE_RDSEED
+#elif defined USE_RDRAND || defined USE_RDSEED || defined USE_DARN
 // Hardware instructions might be available, but use cpuid checks at runtime.
 # pragma GCC poison _M_mt
 // If the runtime cpuid checks fail we'll use a linear congruential engine.
@@ -135,6 +137,24 @@ namespace std _GLIBCXX_VISIBILITY(default)
 #endif
 #endif
 
+#ifdef USE_DARN
+    unsigned int
+    __attribute__((target("cpu=power9")))
+    __ppc_darn(void*)
+    {
+      const uint64_t failed = -1;
+      unsigned int retries = 100;
+      uint64_t val = __builtin_darn();
+      while (val == failed) [[__unlikely__]]
+	{
+	  if (--retries == 0)
+	    std::__throw_runtime_error(__N("random_device: darn failed"));
+	  val = __builtin_darn();
+	}
+      return (uint32_t)val;
+    }
+#endif
+
 #ifdef _GLIBCXX_USE_CRT_RAND_S
     unsigned int
     __winxp_rand_s(void*)
@@ -193,11 +213,16 @@ namespace std _GLIBCXX_VISIBILITY(default)
     }
 #endif
 
-    enum Which {
-      rand_s = 1, rdseed = 2, rdrand = 4, device_file = 8, prng = 16,
+    enum Which : unsigned {
+      device_file = 1, prng = 2, rand_s = 4,
+      rdseed = 64, rdrand = 128, darn = 256,
       any = 0xffff
     };
 
+    constexpr Which
+    operator|(Which l, Which r) noexcept
+    { return Which(unsigned(l) | unsigned(r)); }
+
     inline Which
     which_source(random_device::result_type (*func [[maybe_unused]])(void*),
 		 void* file [[maybe_unused]])
@@ -221,6 +246,11 @@ namespace std _GLIBCXX_VISIBILITY(default)
 	return rdrand;
 #endif
 
+#ifdef USE_DARN
+      if (func == &__ppc_darn)
+	return darn;
+#endif
+
 #ifdef _GLIBCXX_USE_DEV_RANDOM
       if (file != nullptr)
 	return device_file;
@@ -269,6 +299,14 @@ namespace std _GLIBCXX_VISIBILITY(default)
     else if (token == "rdrand" || token == "rdrnd")
       which = rdrand;
 #endif // USE_RDRAND
+#ifdef USE_DARN
+    else if (token == "darn")
+      which = darn;
+#endif
+#if defined USE_RDRAND || defined USE_RDSEED || defined USE_DARN
+    else if (token == "hw" || token == "hardware")
+      which = rdrand | rdseed | darn;
+#endif
 #ifdef _GLIBCXX_USE_CRT_RAND_S
     else if (token == "rand_s")
       which = rand_s;
@@ -346,6 +384,17 @@ namespace std _GLIBCXX_VISIBILITY(default)
     }
 #endif // USE_RDRAND
 
+#ifdef USE_DARN
+    if (which & darn)
+      {
+	if (__builtin_cpu_supports("darn"))
+	  {
+	    _M_func = &__ppc_darn;
+	    return;
+	  }
+      }
+#endif // USE_DARN
+
 #ifdef _GLIBCXX_USE_DEV_RANDOM
     if (which & device_file)
     {
@@ -497,6 +546,7 @@ namespace std _GLIBCXX_VISIBILITY(default)
     {
     case rdrand:
     case rdseed:
+    case darn:
       return (double) max;
     case rand_s:
     case prng:
diff --git a/libstdc++-v3/testsuite/26_numerics/random/random_device/cons/token.cc b/libstdc++-v3/testsuite/26_numerics/random/random_device/cons/token.cc
index aeb7403e830..d6ac3a37c64 100644
--- a/libstdc++-v3/testsuite/26_numerics/random/random_device/cons/token.cc
+++ b/libstdc++-v3/testsuite/26_numerics/random/random_device/cons/token.cc
@@ -51,8 +51,9 @@ test03()
 {
   // At least one of these tokens should be valid.
   const std::string tokens[] = {
-    "rdseed", "rdrand", "rand_s", "/dev/urandom", "/dev/random", "mt19937",
-    "prng"
+    "rdseed", "rdrand", "darn",
+    "rand_s", "/dev/urandom", "/dev/random",
+    "mt19937", "prng"
   };
   int count = 0;
   for (const std::string& token : tokens)
diff --git a/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc b/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc
index 9ef1538d2bb..6f3ebb1b38e 100644
--- a/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc
+++ b/libstdc++-v3/testsuite/26_numerics/random/random_device/entropy.cc
@@ -22,7 +22,7 @@ test01()
       VERIFY( entropy <= max );
     }
 
-  for (auto token : { "rdrand", "rdseed" })
+  for (auto token : { "rdrand", "rdseed", "darn", "hw" })
     if (__gnu_test::random_device_available(token))
     {
       const double entropy = std::random_device(token).entropy();