Rollup merge of #73724 - CryZe:wasm-saturating-casts, r=alexcrichton

Use WASM's saturating casts if they are available

WebAssembly supports saturating floating point to integer casts behind a target feature. The feature is already available on many browsers. Beginning with 1.45 Rust will start defining the behavior of floating point to integer casts to be saturating as well. For this Rust constructs additional checks on top of the `fptoui` / `fptosi` instructions it emits. Here we introduce the possibility for the codegen backend to construct saturating casts itself and only fall back to constructing the checks ourselves if that is not possible.

Resolves part of #73591

3 files changed, 65 insertions, 2 deletions

diff --git a/src/librustc_codegen_llvm/builder.rs b/src/librustc_codegen_llvm/builder.rs
index ba285b5ef38..89b70dce52c 100644
--- a/src/librustc_codegen_llvm/builder.rs
+++ b/src/librustc_codegen_llvm/builder.rs
@@ -18,6 +18,7 @@ use rustc_data_structures::small_c_str::SmallCStr;
 use rustc_hir::def_id::DefId;
 use rustc_middle::ty::layout::TyAndLayout;
 use rustc_middle::ty::{self, Ty, TyCtxt};
+use rustc_span::sym;
 use rustc_target::abi::{self, Align, Size};
 use rustc_target::spec::{HasTargetSpec, Target};
 use std::borrow::Cow;
@@ -652,6 +653,56 @@ impl BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         unsafe { llvm::LLVMBuildSExt(self.llbuilder, val, dest_ty, UNNAMED) }
     }
 
+    fn fptoui_sat(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> Option<&'ll Value> {
+        // WebAssembly has saturating floating point to integer casts if the
+        // `nontrapping-fptoint` target feature is activated. We'll use those if
+        // they are available.
+        if self.sess().target.target.arch == "wasm32"
+            && self.sess().target_features.contains(&sym::nontrapping_fptoint)
+        {
+            let src_ty = self.cx.val_ty(val);
+            let float_width = self.cx.float_width(src_ty);
+            let int_width = self.cx.int_width(dest_ty);
+            let name = match (int_width, float_width) {
+                (32, 32) => Some("llvm.wasm.trunc.saturate.unsigned.i32.f32"),
+                (32, 64) => Some("llvm.wasm.trunc.saturate.unsigned.i32.f64"),
+                (64, 32) => Some("llvm.wasm.trunc.saturate.unsigned.i64.f32"),
+                (64, 64) => Some("llvm.wasm.trunc.saturate.unsigned.i64.f64"),
+                _ => None,
+            };
+            if let Some(name) = name {
+                let intrinsic = self.get_intrinsic(name);
+                return Some(self.call(intrinsic, &[val], None));
+            }
+        }
+        None
+    }
+
+    fn fptosi_sat(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> Option<&'ll Value> {
+        // WebAssembly has saturating floating point to integer casts if the
+        // `nontrapping-fptoint` target feature is activated. We'll use those if
+        // they are available.
+        if self.sess().target.target.arch == "wasm32"
+            && self.sess().target_features.contains(&sym::nontrapping_fptoint)
+        {
+            let src_ty = self.cx.val_ty(val);
+            let float_width = self.cx.float_width(src_ty);
+            let int_width = self.cx.int_width(dest_ty);
+            let name = match (int_width, float_width) {
+                (32, 32) => Some("llvm.wasm.trunc.saturate.signed.i32.f32"),
+                (32, 64) => Some("llvm.wasm.trunc.saturate.signed.i32.f64"),
+                (64, 32) => Some("llvm.wasm.trunc.saturate.signed.i64.f32"),
+                (64, 64) => Some("llvm.wasm.trunc.saturate.signed.i64.f64"),
+                _ => None,
+            };
+            if let Some(name) = name {
+                let intrinsic = self.get_intrinsic(name);
+                return Some(self.call(intrinsic, &[val], None));
+            }
+        }
+        None
+    }
+
     fn fptoui(&mut self, val: &'ll Value, dest_ty: &'ll Type) -> &'ll Value {
         unsafe { llvm::LLVMBuildFPToUI(self.llbuilder, val, dest_ty, UNNAMED) }
     }
diff --git a/src/librustc_codegen_llvm/context.rs b/src/librustc_codegen_llvm/context.rs
index e9a001bab75..d484e15eb2f 100644
--- a/src/librustc_codegen_llvm/context.rs
+++ b/src/librustc_codegen_llvm/context.rs
@@ -497,6 +497,15 @@ impl CodegenCx<'b, 'tcx> {
             t_v8f64: t_f64, 8;
         }
 
+        ifn!("llvm.wasm.trunc.saturate.unsigned.i32.f32", fn(t_f32) -> t_i32);
+        ifn!("llvm.wasm.trunc.saturate.unsigned.i32.f64", fn(t_f64) -> t_i32);
+        ifn!("llvm.wasm.trunc.saturate.unsigned.i64.f32", fn(t_f32) -> t_i64);
+        ifn!("llvm.wasm.trunc.saturate.unsigned.i64.f64", fn(t_f64) -> t_i64);
+        ifn!("llvm.wasm.trunc.saturate.signed.i32.f32", fn(t_f32) -> t_i32);
+        ifn!("llvm.wasm.trunc.saturate.signed.i32.f64", fn(t_f64) -> t_i32);
+        ifn!("llvm.wasm.trunc.saturate.signed.i64.f32", fn(t_f32) -> t_i64);
+        ifn!("llvm.wasm.trunc.saturate.signed.i64.f64", fn(t_f64) -> t_i64);
+
         ifn!("llvm.trap", fn() -> void);
         ifn!("llvm.debugtrap", fn() -> void);
         ifn!("llvm.frameaddress", fn(t_i32) -> i8p);
diff --git a/src/librustc_codegen_llvm/llvm_util.rs b/src/librustc_codegen_llvm/llvm_util.rs
index 80278bb9f53..2e2ce154410 100644
--- a/src/librustc_codegen_llvm/llvm_util.rs
+++ b/src/librustc_codegen_llvm/llvm_util.rs
@@ -250,8 +250,11 @@ const RISCV_WHITELIST: &[(&str, Option<Symbol>)] = &[
     ("e", Some(sym::riscv_target_feature)),
 ];
 
-const WASM_WHITELIST: &[(&str, Option<Symbol>)] =
-    &[("simd128", Some(sym::wasm_target_feature)), ("atomics", Some(sym::wasm_target_feature))];
+const WASM_WHITELIST: &[(&str, Option<Symbol>)] = &[
+    ("simd128", Some(sym::wasm_target_feature)),
+    ("atomics", Some(sym::wasm_target_feature)),
+    ("nontrapping-fptoint", Some(sym::wasm_target_feature)),
+];
 
 /// When rustdoc is running, provide a list of all known features so that all their respective
 /// primitives may be documented.