Attempt to fix-up clang-cl support

If the user specifies `CC=...` then the previous fallback to `clang`
would fail to work if `clang` wasn't in the path. This is quite likely
on Windows in particular.

Lets just drop the use of preprocessor on Windows and directly code the
one constant that the preprocessor was handling.

Author: nagisa

psm/build.rs

@@ -73,38 +73,28 @@ fn main() {
     }
 
     let mut cfg = cc::Build::new();
-    // There seems to be a bug with clang-cl where using
-    // `/clang:-xassembler-with-cpp` is apparently accepted (ie no warnings
-    // about unused/unknown arguments), but results in the same exact error
-    // as if the flag was not present, so instead we take advantage of the
-    // fact that we're not actually compiling any C/C++ code, only assembling
-    // and can just use clang directly.
-    if cfg
+    let msvc = cfg.get_compiler().is_like_msvc();
+    let clang_cl = cfg
         .get_compiler()
         .path()
         .file_name()
         .and_then(|f| f.to_str())
-        .map(|fname| fname.contains("clang-cl"))
-        .unwrap_or(false)
-    {
-        cfg.compiler("clang");
-    }
-
-    let msvc = cfg.get_compiler().is_like_msvc();
-    let asm =
-        if let Some((asm, canswitch)) = find_assembly(&arch, &endian, &os, &env, msvc) {
-            println!("cargo:rustc-cfg=asm");
-            if canswitch {
-                println!("cargo:rustc-cfg=switchable_stack")
-            }
-            asm
-        } else {
-            println!(
-                "cargo:warning=Target {}-{}-{} has no assembly files!",
-                arch, os, env
-            );
-            return;
-        };
+        .map(|f| f.contains("clang-cl"))
+        .unwrap_or(false);
+    let masm = msvc && !clang_cl;
+    let asm = if let Some((asm, canswitch)) = find_assembly(&arch, &endian, &os, &env, masm) {
+        println!("cargo:rustc-cfg=asm");
+        if canswitch {
+            println!("cargo:rustc-cfg=switchable_stack")
+        }
+        asm
+    } else {
+        println!(
+            "cargo:warning=Target {}-{}-{} has no assembly files!",
+            arch, os, env
+        );
+        return;
+    };
 
     if !msvc {
         cfg.flag("-xassembler-with-cpp");

psm/src/arch/x86_64_windows_gnu.s

@@ -1,5 +1,3 @@
-#include "psm.h"
-
 .text
 
 .def rust_psm_stack_direction
@@ -11,7 +9,7 @@
 rust_psm_stack_direction:
 /* extern "sysv64" fn() -> u8 (%al) */
 .cfi_startproc
-    movb $STACK_DIRECTION_DESCENDING, %al # always descending on x86_64
+    movb $2, %al # always descending on x86_64
     retq
 .cfi_endproc
 

psm/src/arch/x86_windows_gnu.s

@@ -1,8 +1,5 @@
 /* FIXME: this works locally but not on appveyor??!? */
-
-#include "psm.h"
 /* NOTE: fastcall calling convention used on all x86 targets */
-
 .text
 
 .def @rust_psm_stack_direction@0
@@ -14,7 +11,7 @@
 @rust_psm_stack_direction@0:
 /* extern "fastcall" fn() -> u8 (%al) */
 .cfi_startproc
-    movb $STACK_DIRECTION_DESCENDING, %al # always descending on x86_64
+    movb $2, %al # always descending on x86_64
     retl
 .cfi_endproc