compiler: factor Windows x86-32 ABI impl into its own file

While it shares more than zero code with the SysV x86-32 ABI impl,
there is no particular reason to organize wildly different ABIs
using if-else in the same function.

compiler/rustc_target/src/callconv/mod.rs

@@ -31,6 +31,7 @@ mod sparc64;
 mod wasm;
 mod x86;
 mod x86_64;
+mod x86_win32;
 mod x86_win64;
 mod xtensa;
 
@@ -649,7 +650,11 @@ impl<'a, Ty> FnAbi<'a, Ty> {
                 };
                 let reg_struct_return = cx.x86_abi_opt().reg_struct_return;
                 let opts = x86::X86Options { flavor, regparm, reg_struct_return };
-                x86::compute_abi_info(cx, self, opts);
+                if spec.is_like_msvc {
+                    x86_win32::compute_abi_info(cx, self, opts);
+                } else {
+                    x86::compute_abi_info(cx, self, opts);
+                }
             }
             "x86_64" => match abi {
                 ExternAbi::SysV64 { .. } => x86_64::compute_abi_info(cx, self),

compiler/rustc_target/src/callconv/x86.rs

@@ -36,7 +36,7 @@ where
             if t.abi_return_struct_as_int || opts.reg_struct_return {
                 // According to Clang, everyone but MSVC returns single-element
                 // float aggregates directly in a floating-point register.
-                if !t.is_like_msvc && fn_abi.ret.layout.is_single_fp_element(cx) {
+                if fn_abi.ret.layout.is_single_fp_element(cx) {
                     match fn_abi.ret.layout.size.bytes() {
                         4 => fn_abi.ret.cast_to(Reg::f32()),
                         8 => fn_abi.ret.cast_to(Reg::f64()),
@@ -64,31 +64,11 @@ where
             continue;
         }
 
-        // FIXME: MSVC 2015+ will pass the first 3 vector arguments in [XYZ]MM0-2
-        // See https://reviews.llvm.org/D72114 for Clang behavior
-
         let t = cx.target_spec();
         let align_4 = Align::from_bytes(4).unwrap();
         let align_16 = Align::from_bytes(16).unwrap();
 
-        if t.is_like_msvc
+        if arg.layout.is_aggregate() {
-            && arg.layout.is_adt()
-            && let Some(max_repr_align) = arg.layout.max_repr_align
-            && max_repr_align > align_4
-        {
-            // MSVC has special rules for overaligned arguments: https://reviews.llvm.org/D72114.
-            // Summarized here:
-            // - Arguments with _requested_ alignment > 4 are passed indirectly.
-            // - For backwards compatibility, arguments with natural alignment > 4 are still passed
-            //   on stack (via `byval`). For example, this includes `double`, `int64_t`,
-            //   and structs containing them, provided they lack an explicit alignment attribute.
-            assert!(
-                arg.layout.align.abi >= max_repr_align,
-                "abi alignment {:?} less than requested alignment {max_repr_align:?}",
-                arg.layout.align.abi,
-            );
-            arg.make_indirect();
-        } else if arg.layout.is_aggregate() {
             // We need to compute the alignment of the `byval` argument. The rules can be found in
             // `X86_32ABIInfo::getTypeStackAlignInBytes` in Clang's `TargetInfo.cpp`. Summarized
             // here, they are:

compiler/rustc_target/src/callconv/x86_win32.rs

@@ -0,0 +1,81 @@
+use rustc_abi::{Align, HasDataLayout, Reg, TyAbiInterface};
+
+use crate::callconv::FnAbi;
+use crate::spec::HasTargetSpec;
+
+pub(crate) fn compute_abi_info<'a, Ty, C>(
+    cx: &C,
+    fn_abi: &mut FnAbi<'a, Ty>,
+    opts: super::x86::X86Options,
+) where
+    Ty: TyAbiInterface<'a, C> + Copy,
+    C: HasDataLayout + HasTargetSpec,
+{
+    if !fn_abi.ret.is_ignore() {
+        if fn_abi.ret.layout.is_aggregate() && fn_abi.ret.layout.is_sized() {
+            // Returning a structure. Most often, this will use
+            // a hidden first argument. On some platforms, though,
+            // small structs are returned as integers.
+            //
+            // Some links:
+            // https://www.angelcode.com/dev/callconv/callconv.html
+            // Clang's ABI handling is in lib/CodeGen/TargetInfo.cpp
+            let t = cx.target_spec();
+            // MSVC does not special-case 1-element float aggregates, unlike others.
+            // GCC used to apply the SysV rule here, breaking windows-gnu's ABI, but was fixed:
+            // - reported in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82028
+            // - fixed in https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85667
+            if t.abi_return_struct_as_int || opts.reg_struct_return {
+                match fn_abi.ret.layout.size.bytes() {
+                    1 => fn_abi.ret.cast_to(Reg::i8()),
+                    2 => fn_abi.ret.cast_to(Reg::i16()),
+                    4 => fn_abi.ret.cast_to(Reg::i32()),
+                    8 => fn_abi.ret.cast_to(Reg::i64()),
+                    _ => fn_abi.ret.make_indirect(),
+                }
+            } else {
+                fn_abi.ret.make_indirect();
+            }
+        } else {
+            fn_abi.ret.extend_integer_width_to(32);
+        }
+    }
+
+    for arg in fn_abi.args.iter_mut() {
+        if arg.is_ignore() || !arg.layout.is_sized() {
+            continue;
+        }
+
+        // FIXME: MSVC 2015+ will pass the first 3 vector arguments in [XYZ]MM0-2
+        // See https://reviews.llvm.org/D72114 for Clang behavior
+
+        let align_4 = Align::from_bytes(4).unwrap();
+
+        if arg.layout.is_adt()
+            && let Some(max_repr_align) = arg.layout.max_repr_align
+            && max_repr_align > align_4
+        {
+            // MSVC has special rules for overaligned arguments: https://reviews.llvm.org/D72114.
+            // Summarized here:
+            // - Arguments with _requested_ alignment > 4 are passed indirectly.
+            // - For backwards compatibility, arguments with natural alignment > 4 are still passed
+            //   on stack (via `byval`). For example, this includes `double`, `int64_t`,
+            //   and structs containing them, provided they lack an explicit alignment attribute.
+            assert!(
+                arg.layout.align.abi >= max_repr_align,
+                "abi alignment {:?} less than requested alignment {max_repr_align:?}",
+                arg.layout.align.abi,
+            );
+            arg.make_indirect();
+        } else if arg.layout.is_aggregate() {
+            // Alignment of the `byval` argument.
+            // The rules can be found in `X86_32ABIInfo::getTypeStackAlignInBytes` in Clang's `TargetInfo.cpp`.
+            let byval_align = align_4;
+            arg.pass_by_stack_offset(Some(byval_align));
+        } else {
+            arg.extend_integer_width_to(32);
+        }
+    }
+
+    super::x86::fill_inregs(cx, fn_abi, opts, false);
+}