random: add RandomMixin::randbits with compile-known bits

In many cases, it is known at compile time how many bits are requested from
randbits. Provide a variant of randbits that accepts this number as a template,
to make sure the compiler can make use of this knowledge. This is used immediately
in rand32() and randbool(), and a few further call sites.

Author: sipa

src/addrman.cpp

@@ -776,7 +776,7 @@ std::pair<CAddress, NodeSeconds> AddrManImpl::Select_(bool new_only, std::option
         const AddrInfo& info{it_found->second};
 
         // With probability GetChance() * chance_factor, return the entry.
-        if (insecure_rand.randbits(30) < chance_factor * info.GetChance() * (1 << 30)) {
+        if (insecure_rand.randbits<30>() < chance_factor * info.GetChance() * (1 << 30)) {
             LogPrint(BCLog::ADDRMAN, "Selected %s from %s\n", info.ToStringAddrPort(), search_tried ? "tried" : "new");
             return {info, info.m_last_try};
         }

src/random.cpp

@@ -741,6 +741,6 @@ void RandomInit()
 
 std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval)
 {
-    double unscaled = -std::log1p(GetRand(uint64_t{1} << 48) * -0.0000000000000035527136788 /* -1/2^48 */);
+    double unscaled = -std::log1p(FastRandomContext().randbits<48>() * -0.0000000000000035527136788 /* -1/2^48 */);
     return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);
 }

src/random.h

@@ -223,6 +223,30 @@ class RandomMixin
         return ret & ((uint64_t{1} << bits) - 1);
     }
 
+    /** Same as above, but with compile-time fixed bits count. */
+    template<int Bits>
+    uint64_t randbits() noexcept
+    {
+        static_assert(Bits >= 0 && Bits <= 64);
+        if constexpr (Bits == 64) {
+            return Impl().rand64();
+        } else {
+            uint64_t ret;
+            if (Bits <= bitbuf_size) {
+                ret = bitbuf;
+                bitbuf >>= Bits;
+                bitbuf_size -= Bits;
+            } else {
+                uint64_t gen = Impl().rand64();
+                ret = (gen << bitbuf_size) | bitbuf;
+                bitbuf = gen >> (Bits - bitbuf_size);
+                bitbuf_size = 64 + bitbuf_size - Bits;
+            }
+            constexpr uint64_t MASK = (uint64_t{1} << Bits) - 1;
+            return ret & MASK;
+        }
+    }
+
     /** Generate a random integer in the range [0..range).
      * Precondition: range > 0.
      */
@@ -247,7 +271,7 @@ class RandomMixin
     }
 
     /** Generate a random 32-bit integer. */
-    uint32_t rand32() noexcept { return Impl().randbits(32); }
+    uint32_t rand32() noexcept { return Impl().template randbits<32>(); }
 
     /** generate a random uint256. */
     uint256 rand256() noexcept
@@ -258,7 +282,7 @@ class RandomMixin
     }
 
     /** Generate a random boolean. */
-    bool randbool() noexcept { return Impl().randbits(1); }
+    bool randbool() noexcept { return Impl().template randbits<1>(); }
 
     /** Return the time point advanced by a uniform random duration. */
     template <typename Tp>

src/test/crypto_tests.cpp

@@ -1195,7 +1195,7 @@ BOOST_AUTO_TEST_CASE(muhash_tests)
         uint256 res;
         int table[4];
         for (int i = 0; i < 4; ++i) {
-            table[i] = g_insecure_rand_ctx.randbits(3);
+            table[i] = g_insecure_rand_ctx.randbits<3>();
         }
         for (int order = 0; order < 4; ++order) {
             MuHash3072 acc;
@@ -1215,8 +1215,8 @@ BOOST_AUTO_TEST_CASE(muhash_tests)
             }
         }
 
-        MuHash3072 x = FromInt(g_insecure_rand_ctx.randbits(4)); // x=X
-        MuHash3072 y = FromInt(g_insecure_rand_ctx.randbits(4)); // x=X, y=Y
+        MuHash3072 x = FromInt(g_insecure_rand_ctx.randbits<4>()); // x=X
+        MuHash3072 y = FromInt(g_insecure_rand_ctx.randbits<4>()); // x=X, y=Y
         MuHash3072 z; // x=X, y=Y, z=1
         z *= x; // x=X, y=Y, z=X
         z *= y; // x=X, y=Y, z=X*Y

src/test/random_tests.cpp

@@ -107,7 +107,7 @@ BOOST_AUTO_TEST_CASE(fastrandom_randbits)
 BOOST_AUTO_TEST_CASE(randbits_test)
 {
     FastRandomContext ctx_lens; //!< RNG for producing the lengths requested from ctx_test.
-    FastRandomContext ctx_test; //!< The RNG being tested.
+    FastRandomContext ctx_test1(true), ctx_test2(true); //!< The RNGs being tested.
     int ctx_test_bitsleft{0}; //!< (Assumed value of) ctx_test::bitbuf_len
 
     // Run the entire test 5 times.
@@ -122,7 +122,25 @@ BOOST_AUTO_TEST_CASE(randbits_test)
                 // Decide on a number of bits to request (0 through 64, inclusive; don't use randbits/randrange).
                 int bits = ctx_lens.rand64() % 65;
                 // Generate that many bits.
-                uint64_t gen = ctx_test.randbits(bits);
+                uint64_t gen = ctx_test1.randbits(bits);
+                // For certain bits counts, also test randbits<Bits> and compare.
+                uint64_t gen2;
+                if (bits == 0) {
+                    gen2 = ctx_test2.randbits<0>();
+                } else if (bits == 1) {
+                    gen2 = ctx_test2.randbits<1>();
+                } else if (bits == 7) {
+                    gen2 = ctx_test2.randbits<7>();
+                } else if (bits == 32) {
+                    gen2 = ctx_test2.randbits<32>();
+                } else if (bits == 51) {
+                    gen2 = ctx_test2.randbits<51>();
+                } else if (bits == 64) {
+                    gen2 = ctx_test2.randbits<64>();
+                } else {
+                    gen2 = ctx_test2.randbits(bits);
+                }
+                BOOST_CHECK_EQUAL(gen, gen2);
                 // Make sure the result is in range.
                 if (bits < 64) BOOST_CHECK_EQUAL(gen >> bits, 0);
                 // Mark all the seen bits in the output.

test/sanitizer_suppressions/ubsan

@@ -77,3 +77,4 @@ shift-base:streams.h
 shift-base:FormatHDKeypath
 shift-base:xoroshiro128plusplus.h
 shift-base:RandomMixin<*>::randbits
+shift-base:RandomMixin<*>::randbits<*>