compiler: `rustc_abi::Abi` => `BackendRepr`

The initial naming of "Abi" was an awful mistake, conveying wrong ideas
about how psABIs worked and even more about what the enum meant.
It was only meant to represent the way the value would be described to
a codegen backend as it was lowered to that intermediate representation.
It was never meant to mean anything about the actual psABI handling!
The conflation is because LLVM typically will associate a certain form
with a certain ABI, but even that does not hold when the special cases
that actually exist arise, plus the IR annotations that modify the ABI.

Reframe `rustc_abi::Abi` as the `BackendRepr` of the type, and rename
`BackendRepr::Aggregate` as `BackendRepr::Memory`. Unfortunately, due to
the persistent misunderstandings, this too is now incorrect:
- Scattered ABI-relevant code is entangled with BackendRepr
- We do not always pre-compute a correct BackendRepr that reflects how
  we "actually" want this value to be handled, so we leave the backend
  interface to also inject various special-cases here
- In some cases `BackendRepr::Memory` is a "real" aggregate, but in
  others it is in fact using memory, and in some cases it is a scalar!

Our rustc-to-backend lowering code handles this sort of thing right now.
That will eventually be addressed by lifting duplicated lowering code
to either rustc_codegen_ssa or rustc_target as appropriate.

1 files changed, 11 insertions, 8 deletions

diff --git a/compiler/rustc_mir_transform/src/gvn.rs b/compiler/rustc_mir_transform/src/gvn.rs
index 79c62372df0..8a646d8cbfe 100644
--- a/compiler/rustc_mir_transform/src/gvn.rs
+++ b/compiler/rustc_mir_transform/src/gvn.rs
@@ -85,6 +85,7 @@
 use std::borrow::Cow;
 
 use either::Either;
+use rustc_abi::{self as abi, BackendRepr, FIRST_VARIANT, FieldIdx, Primitive, Size, VariantIdx};
 use rustc_const_eval::const_eval::DummyMachine;
 use rustc_const_eval::interpret::{
     ImmTy, Immediate, InterpCx, MemPlaceMeta, MemoryKind, OpTy, Projectable, Scalar,
@@ -103,7 +104,6 @@ use rustc_middle::ty::layout::{HasParamEnv, LayoutOf};
 use rustc_middle::ty::{self, Ty, TyCtxt};
 use rustc_span::DUMMY_SP;
 use rustc_span::def_id::DefId;
-use rustc_target::abi::{self, Abi, FIRST_VARIANT, FieldIdx, Primitive, Size, VariantIdx};
 use smallvec::SmallVec;
 use tracing::{debug, instrument, trace};
 
@@ -427,7 +427,10 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     };
                     let ptr_imm = Immediate::new_pointer_with_meta(data, meta, &self.ecx);
                     ImmTy::from_immediate(ptr_imm, ty).into()
-                } else if matches!(ty.abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
+                } else if matches!(
+                    ty.backend_repr,
+                    BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..)
+                ) {
                     let dest = self.ecx.allocate(ty, MemoryKind::Stack).discard_err()?;
                     let variant_dest = if let Some(variant) = variant {
                         self.ecx.project_downcast(&dest, variant).discard_err()?
@@ -573,12 +576,12 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
                     // limited transmutes: it only works between types with the same layout, and
                     // cannot transmute pointers to integers.
                     if value.as_mplace_or_imm().is_right() {
-                        let can_transmute = match (value.layout.abi, to.abi) {
-                            (Abi::Scalar(s1), Abi::Scalar(s2)) => {
+                        let can_transmute = match (value.layout.backend_repr, to.backend_repr) {
+                            (BackendRepr::Scalar(s1), BackendRepr::Scalar(s2)) => {
                                 s1.size(&self.ecx) == s2.size(&self.ecx)
                                     && !matches!(s1.primitive(), Primitive::Pointer(..))
                             }
-                            (Abi::ScalarPair(a1, b1), Abi::ScalarPair(a2, b2)) => {
+                            (BackendRepr::ScalarPair(a1, b1), BackendRepr::ScalarPair(a2, b2)) => {
                                 a1.size(&self.ecx) == a2.size(&self.ecx) &&
                                 b1.size(&self.ecx) == b2.size(&self.ecx) &&
                                 // The alignment of the second component determines its offset, so that also needs to match.
@@ -1241,7 +1244,7 @@ impl<'body, 'tcx> VnState<'body, 'tcx> {
 
         let as_bits = |value| {
             let constant = self.evaluated[value].as_ref()?;
-            if layout.abi.is_scalar() {
+            if layout.backend_repr.is_scalar() {
                 let scalar = self.ecx.read_scalar(constant).discard_err()?;
                 scalar.to_bits(constant.layout.size).discard_err()
             } else {
@@ -1497,12 +1500,12 @@ fn op_to_prop_const<'tcx>(
 
     // Do not synthetize too large constants. Codegen will just memcpy them, which we'd like to
     // avoid.
-    if !matches!(op.layout.abi, Abi::Scalar(..) | Abi::ScalarPair(..)) {
+    if !matches!(op.layout.backend_repr, BackendRepr::Scalar(..) | BackendRepr::ScalarPair(..)) {
         return None;
     }
 
     // If this constant has scalar ABI, return it as a `ConstValue::Scalar`.
-    if let Abi::Scalar(abi::Scalar::Initialized { .. }) = op.layout.abi
+    if let BackendRepr::Scalar(abi::Scalar::Initialized { .. }) = op.layout.backend_repr
         && let Some(scalar) = ecx.read_scalar(op).discard_err()
     {
         if !scalar.try_to_scalar_int().is_ok() {