Auto merge of rust-lang#3535 - RalfJung:host-float, r=RalfJung

update host-float comments

Turns out most of these do not have guaranteed precision anyway so it's fine to use host floats (see rust-lang#121793 and rust-lang#118217). The exception are sqrt and mul_add, tracked at rust-lang/miri#3534 and rust-lang/miri#2995.

Author: bors

src/tools/miri/src/shims/foreign_items.rs

@@ -730,7 +730,7 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             => {
                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
                 let f = this.read_scalar(f)?.to_f32()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, these operations do not have guaranteed precision).
                 let f_host = f.to_host();
                 let res = match link_name.as_str() {
                     "cbrtf" => f_host.cbrt(),
@@ -761,7 +761,7 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let f2 = this.read_scalar(f2)?.to_f32()?;
                 // underscore case for windows, here and below
                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, these operations do not have guaranteed precision).
                 let res = match link_name.as_str() {
                     "_hypotf" | "hypotf" => f1.to_host().hypot(f2.to_host()).to_soft(),
                     "atan2f" => f1.to_host().atan2(f2.to_host()).to_soft(),
@@ -787,7 +787,7 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             => {
                 let [f] = this.check_shim(abi, Abi::C { unwind: false }, link_name, args)?;
                 let f = this.read_scalar(f)?.to_f64()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, these operations do not have guaranteed precision).
                 let f_host = f.to_host();
                 let res = match link_name.as_str() {
                     "cbrt" => f_host.cbrt(),
@@ -818,7 +818,7 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let f2 = this.read_scalar(f2)?.to_f64()?;
                 // underscore case for windows, here and below
                 // (see https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/floating-point-primitives?view=vs-2019)
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, these operations do not have guaranteed precision).
                 let res = match link_name.as_str() {
                     "_hypot" | "hypot" => f1.to_host().hypot(f2.to_host()).to_soft(),
                     "atan2" => f1.to_host().atan2(f2.to_host()).to_soft(),
@@ -848,7 +848,7 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let x = this.read_scalar(x)?.to_f32()?;
                 let signp = this.deref_pointer(signp)?;
 
-                // FIXME: Using host floats.
+                // 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]);
@@ -859,7 +859,7 @@ trait EvalContextExtPriv<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let x = this.read_scalar(x)?.to_f64()?;
                 let signp = this.deref_pointer(signp)?;
 
-                // FIXME: Using host floats.
+                // 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]);

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

@@ -193,12 +193,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             => {
                 let [f] = check_arg_count(args)?;
                 let f = this.read_scalar(f)?.to_f32()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, these operations do not have guaranteed precision).
                 let f_host = f.to_host();
                 let res = match intrinsic_name {
                     "sinf32" => f_host.sin(),
                     "cosf32" => f_host.cos(),
-                    "sqrtf32" => f_host.sqrt(),
+                    "sqrtf32" => f_host.sqrt(), // FIXME Using host floats, this should use full-precision soft-floats
                     "expf32" => f_host.exp(),
                     "exp2f32" => f_host.exp2(),
                     "logf32" => f_host.ln(),
@@ -238,12 +238,12 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
             => {
                 let [f] = check_arg_count(args)?;
                 let f = this.read_scalar(f)?.to_f64()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, these operations do not have guaranteed precision).
                 let f_host = f.to_host();
                 let res = match intrinsic_name {
                     "sinf64" => f_host.sin(),
                     "cosf64" => f_host.cos(),
-                    "sqrtf64" => f_host.sqrt(),
+                    "sqrtf64" => f_host.sqrt(), // FIXME Using host floats, this should use full-precision soft-floats
                     "expf64" => f_host.exp(),
                     "exp2f64" => f_host.exp2(),
                     "logf64" => f_host.ln(),
@@ -366,7 +366,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let [f1, f2] = check_arg_count(args)?;
                 let f1 = this.read_scalar(f1)?.to_f32()?;
                 let f2 = this.read_scalar(f2)?.to_f32()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, this operation does not have guaranteed precision).
                 let res = f1.to_host().powf(f2.to_host()).to_soft();
                 let res = this.adjust_nan(res, &[f1, f2]);
                 this.write_scalar(res, dest)?;
@@ -376,7 +376,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let [f1, f2] = check_arg_count(args)?;
                 let f1 = this.read_scalar(f1)?.to_f64()?;
                 let f2 = this.read_scalar(f2)?.to_f64()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, this operation does not have guaranteed precision).
                 let res = f1.to_host().powf(f2.to_host()).to_soft();
                 let res = this.adjust_nan(res, &[f1, f2]);
                 this.write_scalar(res, dest)?;
@@ -386,7 +386,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let [f, i] = check_arg_count(args)?;
                 let f = this.read_scalar(f)?.to_f32()?;
                 let i = this.read_scalar(i)?.to_i32()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, this operation does not have guaranteed precision).
                 let res = f.to_host().powi(i).to_soft();
                 let res = this.adjust_nan(res, &[f]);
                 this.write_scalar(res, dest)?;
@@ -396,7 +396,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                 let [f, i] = check_arg_count(args)?;
                 let f = this.read_scalar(f)?.to_f64()?;
                 let i = this.read_scalar(i)?.to_i32()?;
-                // FIXME: Using host floats.
+                // Using host floats (but it's fine, this operation does not have guaranteed precision).
                 let res = f.to_host().powi(i).to_soft();
                 let res = this.adjust_nan(res, &[f]);
                 this.write_scalar(res, dest)?;

src/tools/miri/src/shims/intrinsics/simd.rs

@@ -99,14 +99,14 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriInterpCxExt<'mir, 'tcx> {
                             let ty::Float(float_ty) = op.layout.ty.kind() else {
                                 span_bug!(this.cur_span(), "{} operand is not a float", intrinsic_name)
                             };
-                            // FIXME using host floats
+                            // Using host floats (but it's fine, these operations do not have guaranteed precision).
                             match float_ty {
                                 FloatTy::F16 => unimplemented!("f16_f128"),
                                 FloatTy::F32 => {
                                     let f = op.to_scalar().to_f32()?;
                                     let f_host = f.to_host();
                                     let res = match host_op {
-                                        "fsqrt" => f_host.sqrt(),
+                                        "fsqrt" => f_host.sqrt(), // FIXME Using host floats, this should use full-precision soft-floats
                                         "fsin" => f_host.sin(),
                                         "fcos" => f_host.cos(),
                                         "fexp" => f_host.exp(),