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Rollup merge of #100653 - cuviper:fptoint_sat, r=michaelwoerister,antoyo

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Move the cast_float_to_int fallback code to GCC

Now that we require at least LLVM 13, that codegen backend is always
using its intrinsic `fptosi.sat` and `fptoui.sat` conversions, so it
doesn't need the manual implementation. However, the GCC backend still
needs it, so we can move all of that code down there.
This commit is contained in:
Dylan DPC 2022-08-30 16:56:09 +05:30 • committed by GitHub
commit c57a932c3f
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6 changed files with 177 additions and 170 deletions
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157
compiler/rustc_codegen_ssa/src/traits/builder.rs
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@ -1,6 +1,5 @@
use super::abi::AbiBuilderMethods;
use super::asm::AsmBuilderMethods;
use super::consts::ConstMethods;
use super::coverageinfo::CoverageInfoBuilderMethods;
use super::debuginfo::DebugInfoBuilderMethods;
use super::intrinsic::IntrinsicCallMethods;
@ -15,7 +14,6 @@ use crate::mir::operand::OperandRef;
use crate::mir::place::PlaceRef;
use crate::MemFlags;
use rustc_apfloat::{ieee, Float, Round, Status};
use rustc_middle::ty::layout::{HasParamEnv, TyAndLayout};
use rustc_middle::ty::Ty;
use rustc_span::Span;
@ -188,8 +186,8 @@ pub trait BuilderMethods<'a, 'tcx>:
fn trunc(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
fn sext(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
fn fptoui_sat(&mut self, val: Self::Value, dest_ty: Self::Type) -> Option<Self::Value>;
fn fptosi_sat(&mut self, val: Self::Value, dest_ty: Self::Type) -> Option<Self::Value>;
fn fptoui_sat(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
fn fptosi_sat(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
fn fptoui(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
fn fptosi(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
fn uitofp(&mut self, val: Self::Value, dest_ty: Self::Type) -> Self::Value;
@ -223,156 +221,7 @@ pub trait BuilderMethods<'a, 'tcx>:
return if signed { self.fptosi(x, dest_ty) } else { self.fptoui(x, dest_ty) };
}
let try_sat_result =
if signed { self.fptosi_sat(x, dest_ty) } else { self.fptoui_sat(x, dest_ty) };
if let Some(try_sat_result) = try_sat_result {
return try_sat_result;
}
let int_width = self.cx().int_width(int_ty);
let float_width = self.cx().float_width(float_ty);
// 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
// safe code (see issue #10184), so we implement a saturating conversion on top of it:
// Semantically, the mathematical value of the input is rounded towards zero to the next
// mathematical integer, and then the result is clamped into the range of the destination
// integer type. Positive and negative infinity are mapped to the maximum and minimum value of
// the destination integer type. NaN is mapped to 0.
//
// Define f_min and f_max as the largest and smallest (finite) floats that are exactly equal to
// a value representable in int_ty.
// They are exactly equal to int_ty::{MIN,MAX} if float_ty has enough significand bits.
// Otherwise, int_ty::MAX must be rounded towards zero, as it is one less than a power of two.
// int_ty::MIN, however, is either zero or a negative power of two and is thus exactly
// representable. Note that this only works if float_ty's exponent range is sufficiently large.
// f16 or 256 bit integers would break this property. Right now the smallest float type is f32
// with exponents ranging up to 127, which is barely enough for i128::MIN = -2^127.
// 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.
let int_max = |signed: bool, int_width: u64| -> u128 {
let shift_amount = 128 - int_width;
if signed { i128::MAX as u128 >> shift_amount } else { u128::MAX >> shift_amount }
};
let int_min = |signed: bool, int_width: u64| -> i128 {
if signed { i128::MIN >> (128 - int_width) } else { 0 }
};
let compute_clamp_bounds_single = |signed: bool, int_width: u64| -> (u128, u128) {
let rounded_min =
ieee::Single::from_i128_r(int_min(signed, int_width), Round::TowardZero);
assert_eq!(rounded_min.status, Status::OK);
let rounded_max =
ieee::Single::from_u128_r(int_max(signed, int_width), Round::TowardZero);
assert!(rounded_max.value.is_finite());
(rounded_min.value.to_bits(), rounded_max.value.to_bits())
};
let compute_clamp_bounds_double = |signed: bool, int_width: u64| -> (u128, u128) {
let rounded_min =
ieee::Double::from_i128_r(int_min(signed, int_width), Round::TowardZero);
assert_eq!(rounded_min.status, Status::OK);
let rounded_max =
ieee::Double::from_u128_r(int_max(signed, int_width), Round::TowardZero);
assert!(rounded_max.value.is_finite());
(rounded_min.value.to_bits(), rounded_max.value.to_bits())
};
// To implement saturation, we perform the following steps:
//
// 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 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, 0 is returned by definition.
// 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], 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 of int_ty::MAX
// 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 correct.
// QED.
let float_bits_to_llval = |bx: &mut Self, bits| {
let bits_llval = match float_width {
32 => bx.cx().const_u32(bits as u32),
64 => bx.cx().const_u64(bits as u64),
n => bug!("unsupported float width {}", n),
};
bx.bitcast(bits_llval, float_ty)
};
let (f_min, f_max) = match float_width {
32 => compute_clamp_bounds_single(signed, int_width),
64 => compute_clamp_bounds_double(signed, int_width),
n => bug!("unsupported float width {}", n),
};
let f_min = float_bits_to_llval(self, f_min);
let f_max = float_bits_to_llval(self, f_max);
let int_max = self.cx().const_uint_big(int_ty, int_max(signed, int_width));
let int_min = self.cx().const_uint_big(int_ty, int_min(signed, int_width) as u128);
let zero = self.cx().const_uint(int_ty, 0);
// If we're working with vectors, constants must be "splatted": the constant is duplicated
// into each lane of the vector. The algorithm stays the same, we are just using the
// same constant across all lanes.
let maybe_splat = |bx: &mut Self, val| {
if bx.cx().type_kind(dest_ty) == TypeKind::Vector {
bx.vector_splat(bx.vector_length(dest_ty), val)
} else {
val
}
};
let f_min = maybe_splat(self, f_min);
let f_max = maybe_splat(self, f_max);
let int_max = maybe_splat(self, int_max);
let int_min = maybe_splat(self, int_min);
let zero = maybe_splat(self, zero);
// Step 1 ...
let fptosui_result = if signed { self.fptosi(x, dest_ty) } else { self.fptoui(x, dest_ty) };
let less_or_nan = self.fcmp(RealPredicate::RealULT, x, f_min);
let greater = self.fcmp(RealPredicate::RealOGT, x, f_max);
// Step 2: We use two comparisons and two selects, with %s1 being the
// result:
// %less_or_nan = fcmp ult %x, %f_min
// %greater = fcmp olt %x, %f_max
// %s0 = select %less_or_nan, int_ty::MIN, %fptosi_result
// %s1 = select %greater, int_ty::MAX, %s0
// Note that %less_or_nan 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: Unordered comparison can be lowered to a "flipped"
// comparison and a negation, and the negation can be merged into the
// select. Therefore, it not necessarily any more expensive than an
// ordered ("normal") comparison. Whether these optimizations will be
// performed is ultimately up to the backend, but at least x86 does
// perform them.
let s0 = self.select(less_or_nan, int_min, fptosui_result);
let s1 = self.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.
if signed {
// LLVM has no isNaN predicate, so we use (x == x) instead
let cmp = self.fcmp(RealPredicate::RealOEQ, x, x);
self.select(cmp, s1, zero)
} else {
s1
}
if signed { self.fptosi_sat(x, dest_ty) } else { self.fptoui_sat(x, dest_ty) }
}
fn icmp(&mut self, op: IntPredicate, lhs: Self::Value, rhs: Self::Value) -> Self::Value;