intrinsics.fmuladdf{32,64}: expose llvm.fmuladd.* semantics

Add intrinsics `fmuladd{f32,f64}`. This computes `(a * b) + c`, to be
fused if the code generator determines that (i) the target instruction
set has support for a fused operation, and (ii) that the fused operation
is more efficient than the equivalent, separate pair of `mul` and `add`
instructions.

https://llvm.org/docs/LangRef.html#llvm-fmuladd-intrinsic

The codegen_cranelift uses the `fma` function from libc, which is a
correct implementation, but without the desired performance semantic. I
think this requires an update to cranelift to expose a suitable
instruction in its IR.

I have not tested with codegen_gcc, but it should behave the same
way (using `fma` from libc).

Author: jedbrown

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -328,6 +328,9 @@ fn codegen_float_intrinsic_call<'tcx>(
         sym::fabsf64 => ("fabs", 1, fx.tcx.types.f64, types::F64),
         sym::fmaf32 => ("fmaf", 3, fx.tcx.types.f32, types::F32),
         sym::fmaf64 => ("fma", 3, fx.tcx.types.f64, types::F64),
+        // FIXME: calling `fma` from libc without FMA target feature uses expensive sofware emulation
+        sym::fmuladdf32 => ("fmaf", 3, fx.tcx.types.f32, types::F32), // TODO: use cranelift intrinsic analogous to llvm.fmuladd.f32
+        sym::fmuladdf64 => ("fma", 3, fx.tcx.types.f64, types::F64), // TODO: use cranelift intrinsic analogous to llvm.fmuladd.f64
         sym::copysignf32 => ("copysignf", 2, fx.tcx.types.f32, types::F32),
         sym::copysignf64 => ("copysign", 2, fx.tcx.types.f64, types::F64),
         sym::floorf32 => ("floorf", 1, fx.tcx.types.f32, types::F32),
@@ -381,7 +384,7 @@ fn codegen_float_intrinsic_call<'tcx>(
 
     let layout = fx.layout_of(ty);
     let res = match intrinsic {
-        sym::fmaf32 | sym::fmaf64 => {
+        sym::fmaf32 | sym::fmaf64 | sym::fmuladdf32 | sym::fmuladdf64 => {
             CValue::by_val(fx.bcx.ins().fma(args[0], args[1], args[2]), layout)
         }
         sym::copysignf32 | sym::copysignf64 => {

compiler/rustc_codegen_gcc/src/intrinsic/mod.rs

@@ -66,6 +66,9 @@ fn get_simple_intrinsic<'gcc, 'tcx>(
         sym::log2f64 => "log2",
         sym::fmaf32 => "fmaf",
         sym::fmaf64 => "fma",
+        // FIXME: calling `fma` from libc without FMA target feature uses expensive sofware emulation
+        sym::fmuladdf32 => "fmaf", // TODO: use gcc intrinsic analogous to llvm.fmuladd.f32
+        sym::fmuladdf64 => "fma",  // TODO: use gcc intrinsic analogous to llvm.fmuladd.f64
         sym::fabsf32 => "fabsf",
         sym::fabsf64 => "fabs",
         sym::minnumf32 => "fminf",

compiler/rustc_codegen_llvm/src/context.rs

@@ -863,6 +863,11 @@ impl<'ll> CodegenCx<'ll, '_> {
         ifn!("llvm.fma.f64", fn(t_f64, t_f64, t_f64) -> t_f64);
         ifn!("llvm.fma.f128", fn(t_f128, t_f128, t_f128) -> t_f128);
 
+        ifn!("llvm.fmuladd.f16", fn(t_f16, t_f16, t_f16) -> t_f16);
+        ifn!("llvm.fmuladd.f32", fn(t_f32, t_f32, t_f32) -> t_f32);
+        ifn!("llvm.fmuladd.f64", fn(t_f64, t_f64, t_f64) -> t_f64);
+        ifn!("llvm.fmuladd.f128", fn(t_f128, t_f128, t_f128) -> t_f128);
+
         ifn!("llvm.fabs.f16", fn(t_f16) -> t_f16);
         ifn!("llvm.fabs.f32", fn(t_f32) -> t_f32);
         ifn!("llvm.fabs.f64", fn(t_f64) -> t_f64);

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -86,6 +86,11 @@ fn get_simple_intrinsic<'ll>(
         sym::fmaf64 => "llvm.fma.f64",
         sym::fmaf128 => "llvm.fma.f128",
 
+        sym::fmuladdf16 => "llvm.fmuladd.f16",
+        sym::fmuladdf32 => "llvm.fmuladd.f32",
+        sym::fmuladdf64 => "llvm.fmuladd.f64",
+        sym::fmuladdf128 => "llvm.fmuladd.f128",
+
         sym::fabsf16 => "llvm.fabs.f16",
         sym::fabsf32 => "llvm.fabs.f32",
         sym::fabsf64 => "llvm.fabs.f64",

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -359,6 +359,19 @@ pub fn check_intrinsic_type(
                 (0, 0, vec![tcx.types.f128, tcx.types.f128, tcx.types.f128], tcx.types.f128)
             }
 
+            sym::fmuladdf16 => {
+                (0, 0, vec![tcx.types.f16, tcx.types.f16, tcx.types.f16], tcx.types.f16)
+            }
+            sym::fmuladdf32 => {
+                (0, 0, vec![tcx.types.f32, tcx.types.f32, tcx.types.f32], tcx.types.f32)
+            }
+            sym::fmuladdf64 => {
+                (0, 0, vec![tcx.types.f64, tcx.types.f64, tcx.types.f64], tcx.types.f64)
+            }
+            sym::fmuladdf128 => {
+                (0, 0, vec![tcx.types.f128, tcx.types.f128, tcx.types.f128], tcx.types.f128)
+            }
+
             sym::fabsf16 => (0, 0, vec![tcx.types.f16], tcx.types.f16),
             sym::fabsf32 => (0, 0, vec![tcx.types.f32], tcx.types.f32),
             sym::fabsf64 => (0, 0, vec![tcx.types.f64], tcx.types.f64),

compiler/rustc_span/src/symbol.rs

@@ -896,6 +896,10 @@ symbols! {
         fmt,
         fmul_algebraic,
         fmul_fast,
+        fmuladdf128,
+        fmuladdf16,
+        fmuladdf32,
+        fmuladdf64,
         fn_align,
         fn_delegation,
         fn_must_use,

library/core/src/intrinsics.rs

@@ -1803,6 +1803,33 @@ extern "rust-intrinsic" {
     #[rustc_nounwind]
     pub fn fmaf128(a: f128, b: f128, c: f128) -> f128;
 
+    /// Returns `a * b + c` for `f32` values, non-deterministically executing
+    /// either a fused multiply-add or two operations with rounding of the
+    /// intermediate result.
+    ///
+    /// The operation is fused if the code generator determines that target
+    /// instruction set has support for a fused operation, and that the fused
+    /// operation is more efficient than the equivalent, separate pair of mul
+    /// and add instructions. It is unspecified whether or not a fused operation
+    /// is selected, and that may depend on optimization level and context, for
+    /// example.
+    #[rustc_nounwind]
+    #[cfg(not(bootstrap))]
+    pub fn fmuladdf32(a: f32, b: f32, c: f32) -> f32;
+    /// Returns `a * b + c` for `f64` values, non-deterministically executing
+    /// either a fused multiply-add or two operations with rounding of the
+    /// intermediate result.
+    ///
+    /// The operation is fused if the code generator determines that target
+    /// instruction set has support for a fused operation, and that the fused
+    /// operation is more efficient than the equivalent, separate pair of mul
+    /// and add instructions. It is unspecified whether or not a fused operation
+    /// is selected, and that may depend on optimization level and context, for
+    /// example.
+    #[rustc_nounwind]
+    #[cfg(not(bootstrap))]
+    pub fn fmuladdf64(a: f64, b: f64, c: f64) -> f64;
+
     /// Returns the absolute value of an `f16`.
     ///
     /// The stabilized version of this intrinsic is

src/tools/miri/src/intrinsics/mod.rs

@@ -308,6 +308,37 @@ pub trait EvalContextExt<'tcx>: crate::MiriInterpCxExt<'tcx> {
                 this.write_scalar(res, dest)?;
             }
 
+            "fmuladdf32" => {
+                let [a, b, c] = check_arg_count(args)?;
+                let a = this.read_scalar(a)?.to_f32()?;
+                let b = this.read_scalar(b)?.to_f32()?;
+                let c = this.read_scalar(c)?.to_f32()?;
+                let fuse: bool = this.machine.rng.get_mut().gen();
+                let res = if fuse {
+                    // FIXME: Using host floats, to work around https://github.com/rust-lang/rustc_apfloat/issues/11
+                    a.to_host().mul_add(b.to_host(), c.to_host()).to_soft()
+                } else {
+                    ((a * b).value + c).value
+                };
+                let res = this.adjust_nan(res, &[a, b, c]);
+                this.write_scalar(res, dest)?;
+            }
+            "fmuladdf64" => {
+                let [a, b, c] = check_arg_count(args)?;
+                let a = this.read_scalar(a)?.to_f64()?;
+                let b = this.read_scalar(b)?.to_f64()?;
+                let c = this.read_scalar(c)?.to_f64()?;
+                let fuse: bool = this.machine.rng.get_mut().gen();
+                let res = if fuse {
+                    // FIXME: Using host floats, to work around https://github.com/rust-lang/rustc_apfloat/issues/11
+                    a.to_host().mul_add(b.to_host(), c.to_host()).to_soft()
+                } else {
+                    ((a * b).value + c).value
+                };
+                let res = this.adjust_nan(res, &[a, b, c]);
+                this.write_scalar(res, dest)?;
+            }
+
             "powf32" => {
                 let [f1, f2] = check_arg_count(args)?;
                 let f1 = this.read_scalar(f1)?.to_f32()?;

src/tools/miri/tests/pass/float.rs

@@ -30,6 +30,7 @@ fn main() {
     libm();
     test_fast();
     test_algebraic();
+    test_fmuladd();
 }
 
 trait Float: Copy + PartialEq + Debug {
@@ -1041,3 +1042,20 @@ fn test_algebraic() {
     test_operations_f32(11., 2.);
     test_operations_f32(10., 15.);
 }
+
+fn test_fmuladd() {
+    use std::intrinsics::{fmuladdf32, fmuladdf64};
+
+    #[inline(never)]
+    pub fn test_operations_f32(a: f32, b: f32, c: f32) {
+        assert_approx_eq!(unsafe { fmuladdf32(a, b, c) }, a * b + c);
+    }
+
+    #[inline(never)]
+    pub fn test_operations_f64(a: f64, b: f64, c: f64) {
+        assert_approx_eq!(unsafe { fmuladdf64(a, b, c) }, a * b + c);
+    }
+
+    test_operations_f32(0.1, 0.2, 0.3);
+    test_operations_f64(1.1, 1.2, 1.3);
+}

src/tools/miri/tests/pass/intrinsics/fmuladd_nondeterministic.rs

@@ -0,0 +1,44 @@
+#![feature(core_intrinsics)]
+use std::intrinsics::{fmuladdf32, fmuladdf64};
+
+fn main() {
+    let mut saw_zero = false;
+    let mut saw_nonzero = false;
+    for _ in 0..50 {
+        let a = std::hint::black_box(0.1_f64);
+        let b = std::hint::black_box(0.2);
+        let c = std::hint::black_box(-a * b);
+        // It is unspecified whether the following operation is fused or not. The
+        // following evaluates to 0.0 if unfused, and nonzero (-1.66e-18) if fused.
+        let x = unsafe { fmuladdf64(a, b, c) };
+        if x == 0.0 {
+            saw_zero = true;
+        } else {
+            saw_nonzero = true;
+        }
+    }
+    assert!(
+        saw_zero && saw_nonzero,
+        "`fmuladdf64` failed to be evaluated as both fused and unfused"
+    );
+
+    let mut saw_zero = false;
+    let mut saw_nonzero = false;
+    for _ in 0..50 {
+        let a = std::hint::black_box(0.1_f32);
+        let b = std::hint::black_box(0.2);
+        let c = std::hint::black_box(-a * b);
+        // It is unspecified whether the following operation is fused or not. The
+        // following evaluates to 0.0 if unfused, and nonzero (-8.1956386e-10) if fused.
+        let x = unsafe { fmuladdf32(a, b, c) };
+        if x == 0.0 {
+            saw_zero = true;
+        } else {
+            saw_nonzero = true;
+        }
+    }
+    assert!(
+        saw_zero && saw_nonzero,
+        "`fmuladdf32` failed to be evaluated as both fused and unfused"
+    );
+}