Completely overhaul fuzz testing

adds testing for almost every numerical intrinsic

src/int/mod.rs

@@ -72,6 +72,9 @@ pub trait Int:
     /// Prevents the need for excessive conversions between signed and unsigned
     fn logical_shr(self, other: u32) -> Self;
 
+    /// Absolute difference between two integers.
+    fn abs_diff(self, other: Self) -> Self::UnsignedInt;
+
     // copied from primitive integers, but put in a trait
     fn is_zero(self) -> bool;
     fn max_value() -> Self;
@@ -251,6 +254,10 @@ macro_rules! int_impl {
                 me
             }
 
+            fn abs_diff(self, other: Self) -> Self {
+                (self.wrapping_sub(other) as $ity).wrapping_abs() as $uty
+            }
+
             int_impl_common!($uty, $bits);
         }
 
@@ -274,6 +281,10 @@ macro_rules! int_impl {
                 me as $ity
             }
 
+            fn abs_diff(self, other: Self) -> $uty {
+                self.wrapping_sub(other).wrapping_abs() as $uty
+            }
+
             int_impl_common!($ity, $bits);
         }
     };

testcrate/Cargo.toml

@@ -11,7 +11,7 @@ doctest = false
 [build-dependencies]
 rand = "0.7"
 
-[dev-dependencies]
+[dependencies]
 # For fuzzing tests we want a deterministic seedable RNG. We also eliminate potential
 # problems with system RNGs on the variety of platforms this crate is tested on.
 # `xoshiro128**` is used for its quality, size, and speed at generating `u32` shift amounts.

testcrate/src/lib.rs

@@ -1 +1,258 @@
+//! This crate is for integration testing and fuzz testing of functions in `compiler-builtins`. This
+//! includes publicly documented intrinsics and some internal alternative implementation functions
+//! such as `usize_leading_zeros_riscv` (which are tested because they are configured for
+//! architectures not tested by the CI).
+//!
+//! The general idea is to use a combination of edge case testing and randomized fuzz testing. The
+//! edge case testing is crucial for checking cases like where both inputs are equal or equal to
+//! special values such as `i128::MIN`, which is unlikely for the random fuzzer by itself to
+//! encounter. The randomized fuzz testing is specially designed to cover wide swaths of search
+//! space in as few iterations as possible. See `fuzz_values` in `testcrate/tests/misc.rs` for an
+//! example.
+//!
+//! Some floating point tests are disabled on x86 architectures without SSE, because they do not
+//! have correct rounding.
 #![no_std]
+
+use compiler_builtins::float::Float;
+use compiler_builtins::int::Int;
+
+use rand_xoshiro::rand_core::{RngCore, SeedableRng};
+use rand_xoshiro::Xoshiro128StarStar;
+
+/// Sets the number of fuzz iterations run for most tests. In practice, the vast majority of bugs
+/// are caught by the edge case testers. Most of the remaining bugs triggered by more complex
+/// sequences are caught well within 10_000 fuzz iterations. For classes of algorithms like division
+/// that are vulnerable to rare edge cases, we want 1_000_000 iterations to be more confident. In
+/// practical CI, however, we only want to run the more strenuous test once to catch algorithmic
+/// level bugs, and run the 10_000 iteration test on most targets. Target-dependent bugs are likely
+/// to involve miscompilation and misconfiguration that is likely to break algorithms in quickly
+/// caught ways. We choose to configure `N = 1_000_000` iterations for `x86_64` targets which are
+/// likely to have fast hardware, and run `N = 10_000` for all other targets.
+pub const N: u32 = if cfg!(target_arch = "x86_64") {
+    1_000_000
+} else {
+    10_000
+};
+
+/// Random fuzzing step. When run several times, it results in excellent fuzzing entropy such as:
+/// 11110101010101011110111110011111
+/// 10110101010100001011101011001010
+/// 1000000000000000
+/// 10000000000000110111110000001010
+/// 1111011111111101010101111110101
+/// 101111111110100000000101000000
+/// 10000000110100000000100010101
+/// 1010101010101000
+fn fuzz_step<I: Int>(rng: &mut Xoshiro128StarStar, x: &mut I) {
+    let ones = !I::ZERO;
+    let bit_indexing_mask: u32 = I::BITS - 1;
+    // It happens that all the RNG we need can come from one call. 7 bits are needed to index a
+    // worst case 128 bit integer, and there are 4 indexes that need to be made plus 4 bits for
+    // selecting operations
+    let rng32 = rng.next_u32();
+
+    // Randomly OR, AND, and XOR randomly sized and shifted continuous strings of
+    // ones with `lhs` and `rhs`.
+    let r0 = bit_indexing_mask & rng32;
+    let r1 = bit_indexing_mask & (rng32 >> 7);
+    let mask = ones.wrapping_shl(r0).rotate_left(r1);
+    match (rng32 >> 14) % 4 {
+        0 => *x |= mask,
+        1 => *x &= mask,
+        // both 2 and 3 to make XORs as common as ORs and ANDs combined
+        _ => *x ^= mask,
+    }
+
+    // Alternating ones and zeros (e.x. 0b1010101010101010). This catches second-order
+    // problems that might occur for algorithms with two modes of operation (potentially
+    // there is some invariant that can be broken and maintained via alternating between modes,
+    // breaking the algorithm when it reaches the end).
+    let mut alt_ones = I::ONE;
+    for _ in 0..(I::BITS / 2) {
+        alt_ones <<= 2;
+        alt_ones |= I::ONE;
+    }
+    let r0 = bit_indexing_mask & (rng32 >> 16);
+    let r1 = bit_indexing_mask & (rng32 >> 23);
+    let mask = alt_ones.wrapping_shl(r0).rotate_left(r1);
+    match rng32 >> 30 {
+        0 => *x |= mask,
+        1 => *x &= mask,
+        _ => *x ^= mask,
+    }
+}
+
+// We need macros like this, because `#![no_std]` prevents us from using iterators
+macro_rules! edge_cases {
+    ($I:ident, $case:ident, $inner:block) => {
+        for i0 in 0..$I::FUZZ_NUM {
+            let mask_lo = (!$I::UnsignedInt::ZERO).wrapping_shr($I::FUZZ_LENGTHS[i0] as u32);
+            for i1 in i0..I::FUZZ_NUM {
+                let mask_hi =
+                    (!$I::UnsignedInt::ZERO).wrapping_shl($I::FUZZ_LENGTHS[i1 - i0] as u32);
+                let $case = I::from_unsigned(mask_lo & mask_hi);
+                $inner
+            }
+        }
+    };
+}
+
+/// Feeds a series of fuzzing inputs to `f`. The fuzzer first uses an algorithm designed to find
+/// edge cases, followed by a more random fuzzer that runs `n` times.
+pub fn fuzz<I: Int, F: FnMut(I)>(n: u32, mut f: F) {
+    // edge case tester. Calls `f` 210 times for u128.
+    // zero gets skipped by the loop
+    f(I::ZERO);
+    edge_cases!(I, case, {
+        f(case);
+    });
+
+    // random fuzzer
+    let mut rng = Xoshiro128StarStar::seed_from_u64(0);
+    let mut x: I = Int::ZERO;
+    for _ in 0..n {
+        fuzz_step(&mut rng, &mut x);
+        f(x)
+    }
+}
+
+/// The same as `fuzz`, except `f` has two inputs.
+pub fn fuzz_2<I: Int, F: Fn(I, I)>(n: u32, f: F) {
+    // Check cases where the first and second inputs are zero. Both call `f` 210 times for `u128`.
+    edge_cases!(I, case, {
+        f(I::ZERO, case);
+    });
+    edge_cases!(I, case, {
+        f(case, I::ZERO);
+    });
+    // Nested edge tester. Calls `f` 44100 times for `u128`.
+    edge_cases!(I, case0, {
+        edge_cases!(I, case1, {
+            f(case0, case1);
+        })
+    });
+
+    // random fuzzer
+    let mut rng = Xoshiro128StarStar::seed_from_u64(0);
+    let mut x: I = I::ZERO;
+    let mut y: I = I::ZERO;
+    for _ in 0..n {
+        fuzz_step(&mut rng, &mut x);
+        fuzz_step(&mut rng, &mut y);
+        f(x, y)
+    }
+}
+
+/// Tester for shift functions
+pub fn fuzz_shift<I: Int, F: Fn(I, u32)>(f: F) {
+    // Shift functions are very simple and do not need anything other than shifting a small
+    // set of random patterns for every fuzz length.
+    let mut rng = Xoshiro128StarStar::seed_from_u64(0);
+    let mut x: I = Int::ZERO;
+    for i in 0..I::FUZZ_NUM {
+        fuzz_step(&mut rng, &mut x);
+        f(x, Int::ZERO);
+        f(x, I::FUZZ_LENGTHS[i] as u32);
+    }
+}
+
+fn fuzz_float_step<F: Float>(rng: &mut Xoshiro128StarStar, f: &mut F) {
+    let rng32 = rng.next_u32();
+    // we need to fuzz the different parts of the float separately, because the masking on larger
+    // significands will tend to set the exponent to all ones or all zeros frequently
+
+    // sign bit fuzzing
+    let sign = (rng32 & 1) != 0;
+
+    // exponent fuzzing. Only 4 bits for the selector needed.
+    let ones = (F::Int::ONE << F::EXPONENT_BITS) - F::Int::ONE;
+    let r0 = (rng32 >> 1) % F::EXPONENT_BITS;
+    let r1 = (rng32 >> 5) % F::EXPONENT_BITS;
+    // custom rotate shift. Note that `F::Int` is unsigned, so we can shift right without smearing
+    // the sign bit.
+    let mask = if r1 == 0 {
+        ones.wrapping_shr(r0)
+    } else {
+        let tmp = ones.wrapping_shr(r0);
+        (tmp.wrapping_shl(r1) | tmp.wrapping_shr(F::EXPONENT_BITS - r1)) & ones
+    };
+    let mut exp = (f.repr() & F::EXPONENT_MASK) >> F::SIGNIFICAND_BITS;
+    match (rng32 >> 9) % 4 {
+        0 => exp |= mask,
+        1 => exp &= mask,
+        _ => exp ^= mask,
+    }
+
+    // significand fuzzing
+    let mut sig = f.repr() & F::SIGNIFICAND_MASK;
+    fuzz_step(rng, &mut sig);
+    sig &= F::SIGNIFICAND_MASK;
+
+    *f = F::from_parts(sign, exp, sig);
+}
+
+macro_rules! float_edge_cases {
+    ($F:ident, $case:ident, $inner:block) => {
+        for exponent in [
+            F::Int::ZERO,
+            F::Int::ONE,
+            F::Int::ONE << (F::EXPONENT_BITS / 2),
+            (F::Int::ONE << (F::EXPONENT_BITS - 1)) - F::Int::ONE,
+            F::Int::ONE << (F::EXPONENT_BITS - 1),
+            (F::Int::ONE << (F::EXPONENT_BITS - 1)) + F::Int::ONE,
+            (F::Int::ONE << F::EXPONENT_BITS) - F::Int::ONE,
+        ]
+        .iter()
+        {
+            for significand in [
+                F::Int::ZERO,
+                F::Int::ONE,
+                F::Int::ONE << (F::SIGNIFICAND_BITS / 2),
+                (F::Int::ONE << (F::SIGNIFICAND_BITS - 1)) - F::Int::ONE,
+                F::Int::ONE << (F::SIGNIFICAND_BITS - 1),
+                (F::Int::ONE << (F::SIGNIFICAND_BITS - 1)) + F::Int::ONE,
+                (F::Int::ONE << F::SIGNIFICAND_BITS) - F::Int::ONE,
+            ]
+            .iter()
+            {
+                for sign in [false, true].iter() {
+                    let $case = F::from_parts(*sign, *exponent, *significand);
+                    $inner
+                }
+            }
+        }
+    };
+}
+
+pub fn fuzz_float<F: Float, E: Fn(F)>(n: u32, f: E) {
+    float_edge_cases!(F, case, {
+        f(case);
+    });
+
+    // random fuzzer
+    let mut rng = Xoshiro128StarStar::seed_from_u64(0);
+    let mut x = F::ZERO;
+    for _ in 0..n {
+        fuzz_float_step(&mut rng, &mut x);
+        f(x);
+    }
+}
+
+pub fn fuzz_float_2<F: Float, E: Fn(F, F)>(n: u32, f: E) {
+    float_edge_cases!(F, case0, {
+        float_edge_cases!(F, case1, {
+            f(case0, case1);
+        });
+    });
+
+    // random fuzzer
+    let mut rng = Xoshiro128StarStar::seed_from_u64(0);
+    let mut x = F::ZERO;
+    let mut y = F::ZERO;
+    for _ in 0..n {
+        fuzz_float_step(&mut rng, &mut x);
+        fuzz_float_step(&mut rng, &mut y);
+        f(x, y)
+    }
+}