start: Avoid concatenating strings for inline asm.

For csky, we can just always do the gb initialization. For riscv, the gp code is
temporarily pulled above the main switch until the blocking issue is resolved.

Author: alexrp

lib/std/start.zig

@@ -224,14 +224,27 @@ fn EfiMain(handle: uefi.Handle, system_table: *uefi.tables.SystemTable) callconv
 }
 
 fn _start() callconv(.Naked) noreturn {
+    // It's important that all checks here are comptime since having any kind of non-asm code in a
+    // naked function is asking for trouble with some LLVM backends.
+
     // TODO set Top of Stack on non x86_64-plan9
-    if (native_os == .plan9 and native_arch == .x86_64) {
+    if (comptime native_os == .plan9 and native_arch == .x86_64) {
         // from /sys/src/libc/amd64/main9.s
         std.os.plan9.tos = asm volatile (""
             : [tos] "={rax}" (-> *std.os.plan9.Tos),
         );
     }
 
+    // Move this to the riscv prong below when this is resolved: https://github.com/ziglang/zig/issues/20918
+    if (comptime builtin.cpu.arch.isRISCV() and builtin.zig_backend != .stage2_riscv64) asm volatile (
+        \\ .weak __global_pointer$
+        \\ .hidden __global_pointer$
+        \\ .option push
+        \\ .option norelax
+        \\ lla gp, __global_pointer$
+        \\ .option pop
+    );
+
     // Note that we maintain a very low level of trust with regards to ABI guarantees at this point.
     // We will redundantly align the stack, clear the link register, etc. While e.g. the Linux
     // kernel is usually good about upholding the ABI guarantees, the same cannot be said of dynamic
@@ -275,24 +288,19 @@ fn _start() callconv(.Naked) noreturn {
             \\ and sp, #-16
             \\ b %[posixCallMainAndExit]
             ,
-            // zig fmt: off
             .csky =>
-            if (builtin.position_independent_code)
-                // The CSKY ABI assumes that `gb` is set to the address of the GOT in order for
-                // position-independent code to work. We depend on this in `std.os.linux.start_pie`
-                // to locate `_DYNAMIC` as well.
-                \\ grs t0, 1f
-                \\ 1:
-                \\ lrw gb, 1b@GOTPC
-                \\ addu gb, t0
-            else ""
-            ++
+            // The CSKY ABI assumes that `gb` is set to the address of the GOT in order for
+            // position-independent code to work. We depend on this in `std.os.linux.start_pie`
+            // to locate `_DYNAMIC` as well.
+            \\ grs t0, 1f
+            \\ 1:
+            \\ lrw gb, 1b@GOTPC
+            \\ addu gb, t0
             \\ movi lr, 0
             \\ mov a0, sp
             \\ andi sp, sp, -8
             \\ jmpi %[posixCallMainAndExit]
             ,
-            // zig fmt: on
             .hexagon =>
             // r29 = SP, r30 = FP
             \\ r30 = #0
@@ -308,27 +316,13 @@ fn _start() callconv(.Naked) noreturn {
             \\ bstrins.d $sp, $zero, 3, 0
             \\ b %[posixCallMainAndExit]
             ,
-            // zig fmt: off
             .riscv32, .riscv64 =>
-            // The self-hosted riscv64 backend is not able to assemble this yet.
-            if (builtin.zig_backend != .stage2_riscv64)
-                // The RISC-V ELF ABI assumes that `gp` is set to the value of `__global_pointer$` at
-                // startup in order for GP relaxation to work, even in static builds.
-                \\ .weak __global_pointer$
-                \\ .hidden __global_pointer$
-                \\ .option push
-                \\ .option norelax
-                \\ lla gp, __global_pointer$
-                \\ .option pop
-            else ""
-            ++
             \\ li s0, 0
             \\ li ra, 0
             \\ mv a0, sp
             \\ andi sp, sp, -16
             \\ tail %[posixCallMainAndExit]@plt
             ,
-            // zig fmt: off
             .m68k =>
             // Note that the - 8 is needed because pc in the jsr instruction points into the middle
             // of the jsr instruction. (The lea is 6 bytes, the jsr is 4 bytes.)