asm: Allow using r9 (ARM) and x18 (AArch64) if they are not reserved by

the current target.

compiler/rustc_ast_lowering/src/asm.rs

@@ -64,7 +64,12 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let mut clobber_abis = FxHashMap::default();
         if let Some(asm_arch) = asm_arch {
             for (abi_name, abi_span) in &asm.clobber_abis {
-                match asm::InlineAsmClobberAbi::parse(asm_arch, &self.sess.target, *abi_name) {
+                match asm::InlineAsmClobberAbi::parse(
+                    asm_arch,
+                    |feature| self.sess.target_features.contains(&Symbol::intern(feature)),
+                    &self.sess.target,
+                    *abi_name,
+                ) {
                     Ok(abi) => {
                         // If the abi was already in the list, emit an error
                         match clobber_abis.get(&abi) {

compiler/rustc_codegen_ssa/src/target_features.rs

@@ -36,6 +36,7 @@ const ARM_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[
     // #[target_feature].
     ("thumb-mode", Some(sym::arm_target_feature)),
     ("thumb2", Some(sym::arm_target_feature)),
+    ("reserve-r9", Some(sym::arm_target_feature)),
 ];
 
 const AARCH64_ALLOWED_FEATURES: &[(&str, Option<Symbol>)] = &[

compiler/rustc_target/src/asm/aarch64.rs

@@ -1,4 +1,5 @@
 use super::{InlineAsmArch, InlineAsmType};
+use crate::spec::Target;
 use rustc_macros::HashStable_Generic;
 use std::fmt;
 
@@ -70,6 +71,22 @@ impl AArch64InlineAsmRegClass {
     }
 }
 
+pub fn reserved_x18(
+    _arch: InlineAsmArch,
+    _has_feature: impl FnMut(&str) -> bool,
+    target: &Target,
+) -> Result<(), &'static str> {
+    if target.os == "android"
+        || target.is_like_fuchsia
+        || target.is_like_osx
+        || target.is_like_windows
+    {
+        Err("x18 is a reserved register on this target")
+    } else {
+        Ok(())
+    }
+}
+
 def_regs! {
     AArch64 AArch64InlineAsmReg AArch64InlineAsmRegClass {
         x0: reg = ["x0", "w0"],
@@ -90,6 +107,7 @@ def_regs! {
         x15: reg = ["x15", "w15"],
         x16: reg = ["x16", "w16"],
         x17: reg = ["x17", "w17"],
+        x18: reg = ["x18", "w18"] % reserved_x18,
         x20: reg = ["x20", "w20"],
         x21: reg = ["x21", "w21"],
         x22: reg = ["x22", "w22"],
@@ -149,8 +167,6 @@ def_regs! {
         p14: preg = ["p14"],
         p15: preg = ["p15"],
         ffr: preg = ["ffr"],
-        #error = ["x18", "w18"] =>
-            "x18 is used as a reserved register on some targets and cannot be used as an operand for inline asm",
         #error = ["x19", "w19"] =>
             "x19 is used internally by LLVM and cannot be used as an operand for inline asm",
         #error = ["x29", "w29", "fp", "wfp"] =>

compiler/rustc_target/src/asm/arm.rs

@@ -99,6 +99,22 @@ fn not_thumb1(
     }
 }
 
+fn reserved_r9(
+    arch: InlineAsmArch,
+    mut has_feature: impl FnMut(&str) -> bool,
+    target: &Target,
+) -> Result<(), &'static str> {
+    not_thumb1(arch, &mut has_feature, target)?;
+
+    // We detect this using the reserved-r9 feature instead of using the target
+    // because the relocation model can be changed with compiler options.
+    if has_feature("reserved-r9") {
+        Err("the RWPI static base register (r9) cannot be used as an operand for inline asm")
+    } else {
+        Ok(())
+    }
+}
+
 def_regs! {
     Arm ArmInlineAsmReg ArmInlineAsmRegClass {
         r0: reg = ["r0", "a1"],
@@ -109,6 +125,7 @@ def_regs! {
         r5: reg = ["r5", "v2"],
         r7: reg = ["r7", "v4"] % frame_pointer_r7,
         r8: reg = ["r8", "v5"] % not_thumb1,
+        r9: reg = ["r9", "v6", "rfp"] % reserved_r9,
         r10: reg = ["r10", "sl"] % not_thumb1,
         r11: reg = ["r11", "fp"] % frame_pointer_r11,
         r12: reg = ["r12", "ip"] % not_thumb1,
@@ -195,8 +212,6 @@ def_regs! {
         q15: qreg = ["q15"],
         #error = ["r6", "v3"] =>
             "r6 is used internally by LLVM and cannot be used as an operand for inline asm",
-        #error = ["r9", "v6", "rfp"] =>
-            "r9 is used internally by LLVM and cannot be used as an operand for inline asm",
         #error = ["r13", "sp"] =>
             "the stack pointer cannot be used as an operand for inline asm",
         #error = ["r15", "pc"] =>

compiler/rustc_target/src/asm/mod.rs

@@ -785,6 +785,7 @@ pub enum InlineAsmClobberAbi {
     X86_64SysV,
     Arm,
     AArch64,
+    AArch64NoX18,
     RiscV,
 }
 
@@ -793,6 +794,7 @@ impl InlineAsmClobberAbi {
     /// clobber ABIs for the target.
     pub fn parse(
         arch: InlineAsmArch,
+        has_feature: impl FnMut(&str) -> bool,
         target: &Target,
         name: Symbol,
     ) -> Result<Self, &'static [&'static str]> {
@@ -816,7 +818,13 @@ impl InlineAsmClobberAbi {
                 _ => Err(&["C", "system", "efiapi", "aapcs"]),
             },
             InlineAsmArch::AArch64 => match name {
-                "C" | "system" | "efiapi" => Ok(InlineAsmClobberAbi::AArch64),
+                "C" | "system" | "efiapi" => {
+                    Ok(if aarch64::reserved_x18(arch, has_feature, target).is_err() {
+                        InlineAsmClobberAbi::AArch64NoX18
+                    } else {
+                        InlineAsmClobberAbi::AArch64
+                    })
+                }
                 _ => Err(&["C", "system", "efiapi"]),
             },
             InlineAsmArch::RiscV32 | InlineAsmArch::RiscV64 => match name {
@@ -891,8 +899,25 @@ impl InlineAsmClobberAbi {
                 AArch64 AArch64InlineAsmReg {
                     x0, x1, x2, x3, x4, x5, x6, x7,
                     x8, x9, x10, x11, x12, x13, x14, x15,
-                    // x18 is platform-reserved or temporary, but we exclude it
-                    // here since it is a reserved register.
+                    x16, x17, x18, x30,
+
+                    // Technically the low 64 bits of v8-v15 are preserved, but
+                    // we have no way of expressing this using clobbers.
+                    v0, v1, v2, v3, v4, v5, v6, v7,
+                    v8, v9, v10, v11, v12, v13, v14, v15,
+                    v16, v17, v18, v19, v20, v21, v22, v23,
+                    v24, v25, v26, v27, v28, v29, v30, v31,
+
+                    p0, p1, p2, p3, p4, p5, p6, p7,
+                    p8, p9, p10, p11, p12, p13, p14, p15,
+                    ffr,
+
+                }
+            },
+            InlineAsmClobberAbi::AArch64NoX18 => clobbered_regs! {
+                AArch64 AArch64InlineAsmReg {
+                    x0, x1, x2, x3, x4, x5, x6, x7,
+                    x8, x9, x10, x11, x12, x13, x14, x15,
                     x16, x17, x30,
 
                     // Technically the low 64 bits of v8-v15 are preserved, but
@@ -910,7 +935,8 @@ impl InlineAsmClobberAbi {
             },
             InlineAsmClobberAbi::Arm => clobbered_regs! {
                 Arm ArmInlineAsmReg {
-                    // r9 is platform-reserved and is treated as callee-saved.
+                    // r9 is either platform-reserved or callee-saved. Either
+                    // way we don't need to clobber it.
                     r0, r1, r2, r3, r12, r14,
 
                     // The finest-grained register variant is used here so that