Rollup merge of #92425 - calebzulawski:simd-cast, r=workingjubilee

Improve SIMD casts

* Allows `simd_cast` intrinsic to take `usize` and `isize`
* Adds `simd_as` intrinsic, which is the same as `simd_cast` except for saturating float-to-int conversions (matching the behavior of `as`).

cc `@workingjubilee`

1 files changed, 3 insertions, 147 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
index 679c4576701..68decce82ab 100644
--- a/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/rvalue.rs
@@ -3,11 +3,10 @@ use super::place::PlaceRef;
 use super::{FunctionCx, LocalRef};
 
 use crate::base;
-use crate::common::{self, IntPredicate, RealPredicate};
+use crate::common::{self, IntPredicate};
 use crate::traits::*;
 use crate::MemFlags;
 
-use rustc_apfloat::{ieee, Float, Round, Status};
 use rustc_middle::mir;
 use rustc_middle::ty::cast::{CastTy, IntTy};
 use rustc_middle::ty::layout::{HasTyCtxt, LayoutOf};
@@ -368,10 +367,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                                 bx.inttoptr(usize_llval, ll_t_out)
                             }
                             (CastTy::Float, CastTy::Int(IntTy::I)) => {
-                                cast_float_to_int(&mut bx, true, llval, ll_t_in, ll_t_out)
+                                bx.cast_float_to_int(true, llval, ll_t_out)
                             }
                             (CastTy::Float, CastTy::Int(_)) => {
-                                cast_float_to_int(&mut bx, false, llval, ll_t_in, ll_t_out)
+                                bx.cast_float_to_int(false, llval, ll_t_out)
                             }
                             _ => bug!("unsupported cast: {:?} to {:?}", operand.layout.ty, cast.ty),
                         };
@@ -768,146 +767,3 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         // (*) this is only true if the type is suitable
     }
 }
-
-fn cast_float_to_int<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
-    bx: &mut Bx,
-    signed: bool,
-    x: Bx::Value,
-    float_ty: Bx::Type,
-    int_ty: Bx::Type,
-) -> Bx::Value {
-    if let Some(false) = bx.cx().sess().opts.debugging_opts.saturating_float_casts {
-        return if signed { bx.fptosi(x, int_ty) } else { bx.fptoui(x, int_ty) };
-    }
-
-    let try_sat_result = if signed { bx.fptosi_sat(x, int_ty) } else { bx.fptoui_sat(x, int_ty) };
-    if let Some(try_sat_result) = try_sat_result {
-        return try_sat_result;
-    }
-
-    let int_width = bx.cx().int_width(int_ty);
-    let float_width = bx.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())
-    };
-
-    let mut float_bits_to_llval = |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(f_min);
-    let f_max = float_bits_to_llval(f_max);
-    // 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 int_max = bx.cx().const_uint_big(int_ty, int_max(signed, int_width));
-    let int_min = bx.cx().const_uint_big(int_ty, int_min(signed, int_width) as u128);
-    let zero = bx.cx().const_uint(int_ty, 0);
-
-    // Step 1 ...
-    let fptosui_result = if signed { bx.fptosi(x, int_ty) } else { bx.fptoui(x, int_ty) };
-    let less_or_nan = bx.fcmp(RealPredicate::RealULT, x, f_min);
-    let greater = bx.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 = bx.select(less_or_nan, int_min, fptosui_result);
-    let s1 = bx.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 = bx.fcmp(RealPredicate::RealOEQ, x, x);
-        bx.select(cmp, s1, zero)
-    } else {
-        s1
-    }
-}