x: ValueRef,
float_ty: Type,
int_ty: Type) -> ValueRef {
+ let fptosui_result = if signed {
+ bcx.fptosi(x, int_ty)
+ } else {
+ bcx.fptoui(x, int_ty)
+ };
+
if !bcx.sess().opts.debugging_opts.saturating_float_casts {
- if signed {
- return bcx.fptosi(x, int_ty);
- } else {
- return bcx.fptoui(x, int_ty);
- }
+ return fptosui_result;
}
// LLVM's fpto[su]i returns undef when the input x is infinite, NaN, or does not fit into the
// destination integer type after rounding towards zero. This `undef` value can cause UB in
// On the other hand, f_max works even if int_ty::MAX is greater than float_ty::MAX. Because
// we're rounding towards zero, we just get float_ty::MAX (which is always an integer).
// This already happens today with u128::MAX = 2^128 - 1 > f32::MAX.
- fn compute_clamp_bounds<F: Float>(signed: bool, int_ty: Type) -> (u128, u128, Status) {
+ fn compute_clamp_bounds<F: Float>(signed: bool, int_ty: Type) -> (u128, u128) {
let f_min = if signed {
- let int_min = i128::MIN >> (128 - int_ty.int_width());
- let rounded_min = F::from_i128_r(int_min, Round::TowardZero);
+ let rounded_min = F::from_i128_r(int_min(signed, int_ty), Round::TowardZero);
assert_eq!(rounded_min.status, Status::OK);
rounded_min.value
} else {
let rounded_max = F::from_u128_r(int_max(signed, int_ty), Round::TowardZero);
assert!(rounded_max.value.is_finite());
- (f_min.to_bits(), rounded_max.value.to_bits(), rounded_max.status)
+ (f_min.to_bits(), rounded_max.value.to_bits())
}
fn int_max(signed: bool, int_ty: Type) -> u128 {
let shift_amount = 128 - int_ty.int_width();
u128::MAX >> shift_amount
}
}
- let (f_min, f_max, f_max_status) = match float_ty.float_width() {
+ fn int_min(signed: bool, int_ty: Type) -> i128 {
+ if signed {
+ i128::MIN >> (128 - int_ty.int_width())
+ } else {
+ 0
+ }
+ }
+ let (f_min, f_max) = match float_ty.float_width() {
32 => compute_clamp_bounds::<ieee::Single>(signed, int_ty),
64 => compute_clamp_bounds::<ieee::Double>(signed, int_ty),
n => bug!("unsupported float width {}", n),
};
let f_min = float_bits_to_llval(f_min);
let f_max = float_bits_to_llval(f_max);
- // To implement saturation, we perform the following steps (not all steps are necessary for
- // all combinations of int_ty and float_ty, but we'll deal with that below):
+ // To implement saturation, we perform the following steps:
//
- // 1. Clamp x into the range [f_min, f_max] in such a way that NaN becomes f_min.
- // 2. If x is NaN, replace the result of the clamping with 0.0, otherwise
- // keep the clamping result.
- // 3. Now cast the result of step 2 with fpto[su]i.
- // 4. If x > f_max, return int_ty::MAX, otherwise return the result of step 3.
+ // 1. Cast x to an integer with fpto[su]i. This may result in undef.
+ // 2. Compare x to f_min and f_max, and use the comparison results to select:
+ // a) int_ty::MIN if x < f_min or x is NaN
+ // b) int_ty::MAX if x > f_max
+ // c) the result of fpto[su]i otherwise
+ // 3. If x is NaN, return 0.0, otherwise return the result of step 2.
//
- // This avoids undef because values in range [f_min, f_max] by definition fit into the
- // destination type. More importantly, it correctly implements saturating conversion.
+ // This avoids resulting undef because values in range [f_min, f_max] by definition fit into the
+ // destination type. It creates an undef temporary, but *producing* undef is not UB. Our use of
+ // undef does not introduce any non-determinism either.
+ // More importantly, the above procedure correctly implements saturating conversion.
// Proof (sketch):
- // If x is NaN, step 2 yields 0.0, which is converted to 0 in step 3, and NaN > f_max does
- // not hold in step 4, therefore 0 is returned, as desired.
+ // If x is NaN, 0 is trivially returned.
// Otherwise, x is finite or infinite and thus can be compared with f_min and f_max.
// This yields three cases to consider:
- // (1) if x in [f_min, f_max], steps 1, 2, and 4 do nothing and the result of fpto[su]i
- // is returned, which agrees with saturating conversion for inputs in that range.
- // (2) if x > f_max, then x is larger than int_ty::MAX and step 4 correctly returns
- // int_ty::MAX. This holds even if f_max is rounded (i.e., if f_max < int_ty::MAX)
- // because in those cases, nextUp(f_max) is already larger than int_ty::MAX.
- // (3) if x < f_min, then x is smaller than int_ty::MIN and is clamped to f_min. As shown
- // earlier, f_min exactly equals int_ty::MIN and therefore no fixup analogous to step 4
- // is needed. Instead, step 3 casts f_min to int_ty::MIN and step 4 returns this cast
- // result, as desired.
+ // (1) if x in [f_min, f_max], the result of fpto[su]i is returned, which agrees with
+ // saturating conversion for inputs in that range.
+ // (2) if x > f_max, then x is larger than int_ty::MAX. This holds even if f_max is rounded
+ // (i.e., if f_max < int_ty::MAX) because in those cases, nextUp(f_max) is already larger
+ // than int_ty::MAX. Because x is larger than int_ty::MAX, the return value is correct.
+ // (3) if x < f_min, then x is smaller than int_ty::MIN. As shown earlier, f_min exactly equals
+ // int_ty::MIN and therefore the return value of int_ty::MIN is immediately correct.
// QED.
- // Step 1: Clamping. Computed as:
- // clamped_to_min = if f_min < x { x } else { f_min };
- // clamped_x = if f_max < clamped_to_min { f_max } else { clamped_to_min };
- // Note that for x = NaN, both of the above variables become f_min.
- let clamped_to_min = bcx.select(bcx.fcmp(llvm::RealOLT, f_min, x), x, f_min);
- let clamped_x = bcx.select(
- bcx.fcmp(llvm::RealOLT, f_max, clamped_to_min),
- f_max,
- clamped_to_min
- );
-
- // Step 2: NaN replacement.
- // For unsigned types, f_min == 0.0 and therefore clamped_x is already zero.
+ // Step 1 was already performed above.
+
+ // Step 2: We use two comparisons and two selects, with s1 being the result:
+ // %less = fcmp ult %x, %f_min
+ // %greater = fcmp olt %x, %f_max
+ // %s0 = select %less, int_ty::MIN, %fptosi_result
+ // %s1 = select %greater, int_ty::MAX, %s0
+ // Note that %less uses an *unordered* comparison. This comparison is true if the operands are
+ // not comparable (i.e., if x is NaN). The unordered comparison ensures that s1 becomes
+ // int_ty::MIN if x is NaN.
+ // Performance note: It can be lowered to a flipped comparison and a negation (and the negation
+ // can be merged into the select), so it not necessarily any more expensive than a ordered
+ // ("normal") comparison. Whether these optimizations will be performed is ultimately up to the
+ // backend but at least x86 does that.
+ let less = bcx.fcmp(llvm::RealULT, x, f_min);
+ let greater = bcx.fcmp(llvm::RealOGT, x, f_max);
+ let int_max = C_big_integral(int_ty, int_max(signed, int_ty) as u128);
+ let int_min = C_big_integral(int_ty, int_min(signed, int_ty) as u128);
+ let s0 = bcx.select(less, int_min, fptosui_result);
+ let s1 = bcx.select(greater, int_max, s0);
+
+ // Step 3: NaN replacement.
+ // For unsigned types, the above step already yielded int_ty::MIN == 0 if x is NaN.
// Therefore we only need to execute this step for signed integer types.
- let clamped_x = if signed {
- let zero = match float_ty.float_width() {
- 32 => float_bits_to_llval(ieee::Single::ZERO.to_bits()),
- 64 => float_bits_to_llval(ieee::Double::ZERO.to_bits()),
- n => bug!("unsupported float width {}", n),
- };
+ if signed {
// LLVM has no isNaN predicate, so we use (x == x) instead
- bcx.select(bcx.fcmp(llvm::RealOEQ, x, x), clamped_x, zero)
- } else {
- clamped_x
- };
-
- // Step 3: fpto[su]i cast
- let cast_result = if signed {
- bcx.fptosi(clamped_x, int_ty)
- } else {
- bcx.fptoui(clamped_x, int_ty)
- };
-
- // Step 4: f_max fixup.
- // Note that x > f_max implies that x was clamped to f_max in step 1, and therefore the
- // cast result is the integer equal to f_max. If the conversion from int_ty::MAX to f_max
- // was exact, then the result of casting f_max is again int_ty::MAX, so we'd return the same
- // value whether or not x > f_max holds. Therefore, we only need to execute this step
- // if f_max is inexact.
- if f_max_status.contains(Status::INEXACT) {
- let int_max = C_big_integral(int_ty, int_max(signed, int_ty));
- bcx.select(bcx.fcmp(llvm::RealOGT, x, f_max), int_max, cast_result)
+ bcx.select(bcx.fcmp(llvm::RealOEQ, x, x), s1, C_big_integral(int_ty, 0))
} else {
- cast_result
+ s1
}
}
projects / rust.git / blobdiff