asm: support OpCompositeExtract inference for OpTypeStruct fields. (#803)

Author: eddyb

crates/rustc_codegen_spirv/src/builder/spirv_asm.rs

@@ -16,6 +16,7 @@ use rustc_hir::LlvmInlineAsmInner;
 use rustc_middle::bug;
 use rustc_span::{Span, DUMMY_SP};
 use rustc_target::asm::{InlineAsmRegClass, InlineAsmRegOrRegClass, SpirVInlineAsmRegClass};
+use std::convert::TryFrom;
 
 pub struct InstructionTable {
     table: FxHashMap<&'static str, &'static rspirv::grammar::Instruction<'static>>,
@@ -698,13 +699,23 @@ impl<'cx, 'tcx> Builder<'cx, 'tcx> {
 
                 TyPat::IndexComposite(pat) => {
                     let mut ty = subst_ty_pat(cx, pat, ty_vars, leftover_operands)?;
-                    for _index in leftover_operands {
-                        // FIXME(eddyb) support more than just arrays, by looking
-                        // up the indices (of struct fields) as constant integers.
+                    for index in leftover_operands {
+                        let index_to_usize = || match *index {
+                            // FIXME(eddyb) support more than just literals,
+                            // by looking up `IdRef`s as constant integers.
+                            dr::Operand::LiteralInt32(i) => usize::try_from(i).ok(),
+
+                            _ => None,
+                        };
                         ty = match cx.lookup_type(ty) {
                             SpirvType::Array { element, .. }
                             | SpirvType::RuntimeArray { element } => element,
 
+                            SpirvType::Adt { field_types, .. } => *index_to_usize()
+                                .and_then(|i| field_types.get(i))
+                                .ok_or(Ambiguous)?,
+
+                            // FIXME(eddyb) support more than just arrays and structs.
                             _ => return Err(Ambiguous),
                         };
                     }

tests/ui/lang/asm/infer-access-chain-slice.rs

@@ -0,0 +1,32 @@
+// Tests that `asm!` can infer the result type of `OpAccessChain`,
+// when used to index slices.
+
+// build-pass
+
+use spirv_std as _;
+
+use glam::Vec4;
+
+#[spirv(fragment)]
+pub fn main(
+    #[spirv(storage_buffer, descriptor_set = 0, binding = 0)] slice_in: &[Vec4],
+    i: u32,
+    out: &mut Vec4,
+) {
+    unsafe {
+        asm!(
+            // HACK(eddyb) we can't pass in the `&[T]` to `asm!` directly,
+            // and `as *const T` casts would require some special-casing
+            // to avoid actually going through an `OpTypePointer T`, so
+            // instead we extract the data pointer in the `asm!` itself.
+            "%slice_ptr = OpLoad _ {slice_ptr_ptr}",
+            "%data_ptr = OpCompositeExtract _ %slice_ptr 0",
+            "%val_ptr = OpAccessChain _ %data_ptr {index}",
+            "%val = OpLoad _ %val_ptr",
+            "OpStore {out_ptr} %val",
+            slice_ptr_ptr = in(reg) &slice_in,
+            index = in(reg) i,
+            out_ptr = in(reg) out,
+        );
+    }
+}