Make black_box a no-op

Part of #312

Author: LegNeato

crates/rustc_codegen_spirv/src/builder/intrinsics.rs

@@ -9,7 +9,7 @@ use crate::custom_insts::CustomInst;
 use crate::spirv_type::SpirvType;
 use rspirv::dr::Operand;
 use rspirv::spirv::GLOp;
-use rustc_codegen_ssa::mir::operand::OperandRef;
+use rustc_codegen_ssa::mir::operand::{OperandRef, OperandValue};
 use rustc_codegen_ssa::mir::place::PlaceRef;
 use rustc_codegen_ssa::traits::{BuilderMethods, IntrinsicCallBuilderMethods};
 use rustc_middle::ty::layout::LayoutOf;
@@ -240,6 +240,26 @@ impl<'a, 'tcx> IntrinsicCallBuilderMethods<'tcx> for Builder<'a, 'tcx> {
 
             sym::ctpop => self.count_ones(args[0].immediate()),
             sym::bitreverse => self.bit_reverse(args[0].immediate()),
+            sym::black_box => {
+                // TODO(LegNeato): do something more sophisticated that prevents DCE
+                self.tcx
+                    .dcx()
+                    .warn("black_box intrinsic does not prevent optimization in Rust GPU");
+                match args[0].val {
+                    // Return the value unchanged for immediates
+                    OperandValue::Immediate(v) | OperandValue::Pair(v, _) => v,
+                    // For non-immediates, store and reload
+                    OperandValue::Ref(place) => self.load(
+                        self.layout_of(arg_tys[0]).spirv_type(self.span(), self),
+                        place.llval,
+                        place.align,
+                    ),
+                    // For ZSTs, return an undef value that will be optimized away
+                    OperandValue::ZeroSized => {
+                        self.undef(self.layout_of(arg_tys[0]).spirv_type(self.span(), self))
+                    }
+                }
+            }
             sym::bswap => {
                 // https://github.com/KhronosGroup/SPIRV-LLVM/pull/221/files
                 // TODO: Definitely add tests to make sure this impl is right.

tests/compiletests/ui/lang/core/intrinsics/black_box.rs

@@ -0,0 +1,29 @@
+// Test black_box intrinsic
+// build-pass
+
+#![allow(internal_features)]
+#![feature(core_intrinsics)]
+#![no_std]
+
+use core::hint::black_box;
+use spirv_std::spirv;
+
+#[spirv(compute(threads(1)))]
+pub fn main() {
+    // Test with various types
+    let x = 42i32;
+    let y = black_box(x);
+
+    let a = 3.14f32;
+    let b = black_box(a);
+
+    let v = [1, 2, 3, 4];
+    let w = black_box(v);
+
+    // Test in expressions
+    let result = black_box(10) + black_box(20);
+
+    // Test with references
+    let data = 100u32;
+    let ref_data = black_box(&data);
+}

tests/compiletests/ui/lang/core/intrinsics/black_box.stderr

@@ -0,0 +1,4 @@
+warning: black_box intrinsic does not prevent optimization in Rust GPU
+
+warning: 1 warning emitted
+