Disallow the use of high byte registes as operands on x86_64

They are still allowed on x86 though.

Fixes #83495

4 files changed, 22 insertions, 13 deletions

diff --git a/src/doc/unstable-book/src/library-features/asm.md b/src/doc/unstable-book/src/library-features/asm.md
index c0e23b834d1..6c61f4f0d20 100644
--- a/src/doc/unstable-book/src/library-features/asm.md
+++ b/src/doc/unstable-book/src/library-features/asm.md
@@ -495,7 +495,7 @@ Here is the list of currently supported register classes:
 | x86 | `reg` | `ax`, `bx`, `cx`, `dx`, `si`, `di`, `r[8-15]` (x86-64 only) | `r` |
 | x86 | `reg_abcd` | `ax`, `bx`, `cx`, `dx` | `Q` |
 | x86-32 | `reg_byte` | `al`, `bl`, `cl`, `dl`, `ah`, `bh`, `ch`, `dh` | `q` |
-| x86-64 | `reg_byte` | `al`, `bl`, `cl`, `dl`, `sil`, `dil`, `r[8-15]b`, `ah`\*, `bh`\*, `ch`\*, `dh`\* | `q` |
+| x86-64 | `reg_byte`\* | `al`, `bl`, `cl`, `dl`, `sil`, `dil`, `r[8-15]b` | `q` |
 | x86 | `xmm_reg` | `xmm[0-7]` (x86) `xmm[0-15]` (x86-64) | `x` |
 | x86 | `ymm_reg` | `ymm[0-7]` (x86) `ymm[0-15]` (x86-64) | `x` |
 | x86 | `zmm_reg` | `zmm[0-7]` (x86) `zmm[0-31]` (x86-64) | `v` |
@@ -526,7 +526,7 @@ Here is the list of currently supported register classes:
 
 > **Note**: On x86 we treat `reg_byte` differently from `reg` because the compiler can allocate `al` and `ah` separately whereas `reg` reserves the whole register.
 >
-> Note #2: On x86-64 the high byte registers (e.g. `ah`) are only available when used as an explicit register. Specifying the `reg_byte` register class for an operand will always allocate a low byte register.
+> Note #2: On x86-64 the high byte registers (e.g. `ah`) are not available in the `reg_byte` register class.
 >
 > Note #3: NVPTX doesn't have a fixed register set, so named registers are not supported.
 >
diff --git a/src/test/assembly/asm/x86-types.rs b/src/test/assembly/asm/x86-types.rs
index e0190d3bdae..b65b727d225 100644
--- a/src/test/assembly/asm/x86-types.rs
+++ b/src/test/assembly/asm/x86-types.rs
@@ -748,10 +748,11 @@ check_reg!(eax_f64 f64 "eax" "mov");
 // CHECK: #NO_APP
 check_reg!(eax_ptr ptr "eax" "mov");
 
-// CHECK-LABEL: ah_byte:
-// CHECK: #APP
-// CHECK: mov ah, ah
-// CHECK: #NO_APP
+// i686-LABEL: ah_byte:
+// i686: #APP
+// i686: mov ah, ah
+// i686: #NO_APP
+#[cfg(i686)]
 check_reg!(ah_byte i8 "ah" "mov");
 
 // CHECK-LABEL: xmm0_i32:
diff --git a/src/test/ui/asm/bad-reg.rs b/src/test/ui/asm/bad-reg.rs
index 016ea9329c4..da302b24876 100644
--- a/src/test/ui/asm/bad-reg.rs
+++ b/src/test/ui/asm/bad-reg.rs
@@ -37,6 +37,8 @@ fn main() {
         //~^ ERROR invalid register `mm0`: MMX registers are not currently supported as operands
         asm!("", in("k0") foo);
         //~^ ERROR invalid register `k0`: the k0 AVX mask register cannot be used as an operand
+        asm!("", in("ah") foo);
+        //~^ ERROR invalid register `ah`: high byte registers cannot be used as an operand
 
         // Explicit register conflicts
         // (except in/lateout which don't conflict)
diff --git a/src/test/ui/asm/bad-reg.stderr b/src/test/ui/asm/bad-reg.stderr
index c6b7d310dfa..2bfb4854c34 100644
--- a/src/test/ui/asm/bad-reg.stderr
+++ b/src/test/ui/asm/bad-reg.stderr
@@ -94,8 +94,14 @@ error: invalid register `k0`: the k0 AVX mask register cannot be used as an oper
 LL |         asm!("", in("k0") foo);
    |                  ^^^^^^^^^^^^
 
+error: invalid register `ah`: high byte registers cannot be used as an operand on x86_64
+  --> $DIR/bad-reg.rs:40:18
+   |
+LL |         asm!("", in("ah") foo);
+   |                  ^^^^^^^^^^^^
+
 error: register `al` conflicts with register `ax`
-  --> $DIR/bad-reg.rs:44:33
+  --> $DIR/bad-reg.rs:46:33
    |
 LL |         asm!("", in("eax") foo, in("al") bar);
    |                  -------------  ^^^^^^^^^^^^ register `al`
@@ -103,7 +109,7 @@ LL |         asm!("", in("eax") foo, in("al") bar);
    |                  register `ax`
 
 error: register `ax` conflicts with register `ax`
-  --> $DIR/bad-reg.rs:46:33
+  --> $DIR/bad-reg.rs:48:33
    |
 LL |         asm!("", in("rax") foo, out("rax") bar);
    |                  -------------  ^^^^^^^^^^^^^^ register `ax`
@@ -111,13 +117,13 @@ LL |         asm!("", in("rax") foo, out("rax") bar);
    |                  register `ax`
    |
 help: use `lateout` instead of `out` to avoid conflict
-  --> $DIR/bad-reg.rs:46:18
+  --> $DIR/bad-reg.rs:48:18
    |
 LL |         asm!("", in("rax") foo, out("rax") bar);
    |                  ^^^^^^^^^^^^^
 
 error: register `ymm0` conflicts with register `xmm0`
-  --> $DIR/bad-reg.rs:49:34
+  --> $DIR/bad-reg.rs:51:34
    |
 LL |         asm!("", in("xmm0") foo, in("ymm0") bar);
    |                  --------------  ^^^^^^^^^^^^^^ register `ymm0`
@@ -125,7 +131,7 @@ LL |         asm!("", in("xmm0") foo, in("ymm0") bar);
    |                  register `xmm0`
 
 error: register `ymm0` conflicts with register `xmm0`
-  --> $DIR/bad-reg.rs:51:34
+  --> $DIR/bad-reg.rs:53:34
    |
 LL |         asm!("", in("xmm0") foo, out("ymm0") bar);
    |                  --------------  ^^^^^^^^^^^^^^^ register `ymm0`
@@ -133,10 +139,10 @@ LL |         asm!("", in("xmm0") foo, out("ymm0") bar);
    |                  register `xmm0`
    |
 help: use `lateout` instead of `out` to avoid conflict
-  --> $DIR/bad-reg.rs:51:18
+  --> $DIR/bad-reg.rs:53:18
    |
 LL |         asm!("", in("xmm0") foo, out("ymm0") bar);
    |                  ^^^^^^^^^^^^^^
 
-error: aborting due to 18 previous errors
+error: aborting due to 19 previous errors