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Merged
merged 2 commits into from
Feb 16, 2025
Merged

Apply random float error #4156

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f5330d0
apply random float error to most floating-point operations
LorrensP-2158466 Jan 12, 2025
5c9eaa3
add erf and erfc to nondet tests, and reduce how much we're changing …
RalfJung Feb 16, 2025
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113 changes: 89 additions & 24 deletions src/intrinsics/mod.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -7,12 +7,13 @@ use rand::Rng;
use rustc_abi::Size;
use rustc_apfloat::{Float, Round};
use rustc_middle::mir;
use rustc_middle::ty::{self, FloatTy};
use rustc_middle::ty::{self, FloatTy, ScalarInt};
use rustc_span::{Symbol, sym};

use self::atomic::EvalContextExt as _;
use self::helpers::{ToHost, ToSoft, check_intrinsic_arg_count};
use self::simd::EvalContextExt as _;
use crate::math::apply_random_float_error_ulp;
use crate::*;

impl<'tcx> EvalContextExt<'tcx> for crate::MiriInterpCx<'tcx> {}
Expand Down Expand Up @@ -206,10 +207,26 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
this.write_scalar(res, dest)?;
}

"sqrtf32" => {
let [f] = check_intrinsic_arg_count(args)?;
let f = this.read_scalar(f)?.to_f32()?;
// Sqrt is specified to be fully precise.
let res = math::sqrt(f);
let res = this.adjust_nan(res, &[f]);
this.write_scalar(res, dest)?;
}
"sqrtf64" => {
let [f] = check_intrinsic_arg_count(args)?;
let f = this.read_scalar(f)?.to_f64()?;
// Sqrt is specified to be fully precise.
let res = math::sqrt(f);
let res = this.adjust_nan(res, &[f]);
this.write_scalar(res, dest)?;
}

#[rustfmt::skip]
| "sinf32"
| "cosf32"
| "sqrtf32"
| "expf32"
| "exp2f32"
| "logf32"
Expand All @@ -218,26 +235,33 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
=> {
let [f] = check_intrinsic_arg_count(args)?;
let f = this.read_scalar(f)?.to_f32()?;
// Using host floats except for sqrt (but it's fine, these operations do not have
// Using host floats (but it's fine, these operations do not have
// guaranteed precision).
let host = f.to_host();
let res = match intrinsic_name {
"sinf32" => f.to_host().sin().to_soft(),
"cosf32" => f.to_host().cos().to_soft(),
"sqrtf32" => math::sqrt(f),
"expf32" => f.to_host().exp().to_soft(),
"exp2f32" => f.to_host().exp2().to_soft(),
"logf32" => f.to_host().ln().to_soft(),
"log10f32" => f.to_host().log10().to_soft(),
"log2f32" => f.to_host().log2().to_soft(),
"sinf32" => host.sin(),
"cosf32" => host.cos(),
"expf32" => host.exp(),
"exp2f32" => host.exp2(),
"logf32" => host.ln(),
"log10f32" => host.log10(),
"log2f32" => host.log2(),
_ => bug!(),
};
let res = res.to_soft();
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res = apply_random_float_error_ulp(
this,
res,
4, // log2(16)
);
let res = this.adjust_nan(res, &[f]);
this.write_scalar(res, dest)?;
}
#[rustfmt::skip]
| "sinf64"
| "cosf64"
| "sqrtf64"
| "expf64"
| "exp2f64"
| "logf64"
Expand All @@ -246,19 +270,27 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
=> {
let [f] = check_intrinsic_arg_count(args)?;
let f = this.read_scalar(f)?.to_f64()?;
// Using host floats except for sqrt (but it's fine, these operations do not have
// Using host floats (but it's fine, these operations do not have
// guaranteed precision).
let host = f.to_host();
let res = match intrinsic_name {
"sinf64" => f.to_host().sin().to_soft(),
"cosf64" => f.to_host().cos().to_soft(),
"sqrtf64" => math::sqrt(f),
"expf64" => f.to_host().exp().to_soft(),
"exp2f64" => f.to_host().exp2().to_soft(),
"logf64" => f.to_host().ln().to_soft(),
"log10f64" => f.to_host().log10().to_soft(),
"log2f64" => f.to_host().log2().to_soft(),
"sinf64" => host.sin(),
"cosf64" => host.cos(),
"expf64" => host.exp(),
"exp2f64" => host.exp2(),
"logf64" => host.ln(),
"log10f64" => host.log10(),
"log2f64" => host.log2(),
_ => bug!(),
};
let res = res.to_soft();
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res = apply_random_float_error_ulp(
this,
res,
4, // log2(16)
);
let res = this.adjust_nan(res, &[f]);
this.write_scalar(res, dest)?;
}
Expand Down Expand Up @@ -316,6 +348,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
}

"powf32" => {
// FIXME: apply random relative error but without altering behaviour of powf
let [f1, f2] = check_intrinsic_arg_count(args)?;
let f1 = this.read_scalar(f1)?.to_f32()?;
let f2 = this.read_scalar(f2)?.to_f32()?;
Expand All @@ -325,6 +358,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
this.write_scalar(res, dest)?;
}
"powf64" => {
// FIXME: apply random relative error but without altering behaviour of powf
let [f1, f2] = check_intrinsic_arg_count(args)?;
let f1 = this.read_scalar(f1)?.to_f64()?;
let f2 = this.read_scalar(f2)?.to_f64()?;
Expand All @@ -335,6 +369,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
}

"powif32" => {
// FIXME: apply random relative error but without altering behaviour of powi
let [f, i] = check_intrinsic_arg_count(args)?;
let f = this.read_scalar(f)?.to_f32()?;
let i = this.read_scalar(i)?.to_i32()?;
Expand All @@ -344,6 +379,7 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
this.write_scalar(res, dest)?;
}
"powif64" => {
// FIXME: apply random relative error but without altering behaviour of powi
let [f, i] = check_intrinsic_arg_count(args)?;
let f = this.read_scalar(f)?.to_f64()?;
let i = this.read_scalar(i)?.to_i32()?;
Expand Down Expand Up @@ -372,7 +408,10 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
_ => bug!(),
};
let res = this.binary_op(op, &a, &b)?;
// `binary_op` already called `generate_nan` if necessary.
// `binary_op` already called `generate_nan` if needed.
// Apply a relative error of 16ULP to simulate non-deterministic precision loss
// due to optimizations.
let res = apply_random_float_error_to_imm(this, res, 4 /* log2(16) */)?;
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@RalfJung Is this really allowed behavior for the algebraic intrinsics? From the documentation for fadd_algebraic "Float addition that allows optimizations based on algebraic rules." I would assume that this should either evaluate a + b or b + a, but not introduce random errors.

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@RalfJung RalfJung May 12, 2025 •
edited
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Yes it is. Note that a + b and b + a produce identical results even for floats, so that optimization can be performed without the program even noticing, and happens even when you do not use the intrinsics. The intrinsic also allows things like associativity and distributivity, which can change the results -- and there's no good way to put any bounds on how it changes the results.

This is a closed PR, not a venue for discussion. If you have further questions, please open a thread on the forum.

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@RalfJung RalfJung May 12, 2025 •
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Note that the docs even call this out:

Because of the unpredictable nature of compiler optimizations, the same inputs may produce different results even within a single program run. Unsafe code must not rely on any property of the return value for soundness.

Anything beyond "returns an arbitrary random float" is a quality-of-implementation concern, not a stable guarantee unsafe code can rely on.

this.write_immediate(*res, dest)?;
}

Expand Down Expand Up @@ -418,11 +457,14 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
_ => {}
}
let res = this.binary_op(op, &a, &b)?;
// This cannot be a NaN so we also don't have to apply any non-determinism.
// (Also, `binary_op` already called `generate_nan` if needed.)
if !float_finite(&res)? {
throw_ub_format!("`{intrinsic_name}` intrinsic produced non-finite value as result");
}
// This cannot be a NaN so we also don't have to apply any non-determinism.
// (Also, `binary_op` already called `generate_nan` if needed.)
// Apply a relative error of 16ULP to simulate non-deterministic precision loss
// due to optimizations.
let res = apply_random_float_error_to_imm(this, res, 4 /* log2(16) */)?;
this.write_immediate(*res, dest)?;
}

Expand Down Expand Up @@ -455,3 +497,26 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
interp_ok(EmulateItemResult::NeedsReturn)
}
}

/// Applies a random 16ULP floating point error to `val` and returns the new value.
/// Will fail if `val` is not a floating point number.
fn apply_random_float_error_to_imm<'tcx>(
ecx: &mut MiriInterpCx<'tcx>,
val: ImmTy<'tcx>,
ulp_exponent: u32,
) -> InterpResult<'tcx, ImmTy<'tcx>> {
let scalar = val.to_scalar_int()?;
let res: ScalarInt = match val.layout.ty.kind() {
ty::Float(FloatTy::F16) =>
apply_random_float_error_ulp(ecx, scalar.to_f16(), ulp_exponent).into(),
ty::Float(FloatTy::F32) =>
apply_random_float_error_ulp(ecx, scalar.to_f32(), ulp_exponent).into(),
ty::Float(FloatTy::F64) =>
apply_random_float_error_ulp(ecx, scalar.to_f64(), ulp_exponent).into(),
ty::Float(FloatTy::F128) =>
apply_random_float_error_ulp(ecx, scalar.to_f128(), ulp_exponent).into(),
_ => bug!("intrinsic called with non-float input type"),
};

interp_ok(ImmTy::from_scalar_int(res, val.layout))
}
16 changes: 16 additions & 0 deletions src/math.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -27,6 +27,22 @@ pub(crate) fn apply_random_float_error<F: rustc_apfloat::Float>(
(val * (F::from_u128(1).value + err).value).value
}

/// [`apply_random_float_error`] gives instructions to apply a 2^N ULP error.
/// This function implements these instructions such that applying a 2^N ULP error is less error prone.
/// So for a 2^N ULP error, you would pass N as the `ulp_exponent` argument.
pub(crate) fn apply_random_float_error_ulp<F: rustc_apfloat::Float>(
ecx: &mut crate::MiriInterpCx<'_>,
val: F,
ulp_exponent: u32,
) -> F {
let n = i32::try_from(ulp_exponent)
.expect("`err_scale_for_ulp`: exponent is too large to create an error scale");
// we know this fits
let prec = i32::try_from(F::PRECISION).unwrap();
let err_scale = -(prec - n - 1);
apply_random_float_error(ecx, val, err_scale)
}

pub(crate) fn sqrt<S: rustc_apfloat::ieee::Semantics>(x: IeeeFloat<S>) -> IeeeFloat<S> {
match x.category() {
// preserve zero sign
Expand Down
42 changes: 38 additions & 4 deletions src/shims/foreign_items.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -765,7 +765,13 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
"erfcf" => f_host.erfc(),
_ => bug!(),
};
let res = res.to_soft();
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res = math::apply_random_float_error_ulp(
this,
res.to_soft(),
4, // log2(16)
);
let res = this.adjust_nan(res, &[f]);
this.write_scalar(res, dest)?;
}
Expand All @@ -788,6 +794,13 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
"fdimf" => f1.to_host().abs_sub(f2.to_host()).to_soft(),
_ => bug!(),
};
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res = math::apply_random_float_error_ulp(
this,
res,
4, // log2(16)
);
let res = this.adjust_nan(res, &[f1, f2]);
this.write_scalar(res, dest)?;
}
Expand Down Expand Up @@ -826,7 +839,13 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
"erfc" => f_host.erfc(),
_ => bug!(),
};
let res = res.to_soft();
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res = math::apply_random_float_error_ulp(
this,
res.to_soft(),
4, // log2(16)
);
let res = this.adjust_nan(res, &[f]);
this.write_scalar(res, dest)?;
}
Expand All @@ -849,6 +868,13 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
"fdim" => f1.to_host().abs_sub(f2.to_host()).to_soft(),
_ => bug!(),
};
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res = math::apply_random_float_error_ulp(
this,
res,
4, // log2(16)
);
let res = this.adjust_nan(res, &[f1, f2]);
this.write_scalar(res, dest)?;
}
Expand All @@ -874,7 +900,11 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
// Using host floats (but it's fine, these operations do not have guaranteed precision).
let (res, sign) = x.to_host().ln_gamma();
this.write_int(sign, &signp)?;
let res = this.adjust_nan(res.to_soft(), &[x]);
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res =
math::apply_random_float_error_ulp(this, res.to_soft(), 4 /* log2(16) */);
let res = this.adjust_nan(res, &[x]);
this.write_scalar(res, dest)?;
}
"lgamma_r" => {
Expand All @@ -885,7 +915,11 @@ trait EvalContextExtPriv<'tcx>: crate::MiriInterpCxExt<'tcx> {
// Using host floats (but it's fine, these operations do not have guaranteed precision).
let (res, sign) = x.to_host().ln_gamma();
this.write_int(sign, &signp)?;
let res = this.adjust_nan(res.to_soft(), &[x]);
// Apply a relative error of 16ULP to introduce some non-determinism
// simulating imprecise implementations and optimizations.
let res =
math::apply_random_float_error_ulp(this, res.to_soft(), 4 /* log2(16) */);
let res = this.adjust_nan(res, &[x]);
this.write_scalar(res, dest)?;
}

Expand Down
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