start: Avoid going through the argc_argv_ptr global variable.

This is problematic for PIE. There's nothing but luck preventing the accesses to
this global variable from requiring relocations. I've observed this being an
issue on MIPS and PowerPC personally, but others may be affected.

Besides, we're really just passing the initial stack pointer value to
posixCallMainAndExit(), so... just do that.

Author: alexrp

lib/std/start.zig

@@ -9,8 +9,6 @@ const elf = std.elf;
 const native_arch = builtin.cpu.arch;
 const native_os = builtin.os.tag;
 
-var argc_argv_ptr: [*]usize = undefined;
-
 const start_sym_name = if (native_arch.isMIPS()) "__start" else "_start";
 
 // The self-hosted compiler is not fully capable of handling all of this start.zig file.
@@ -277,41 +275,42 @@ fn _start() callconv(.Naked) noreturn {
     asm volatile (switch (native_arch) {
             .x86_64 =>
             \\ xorl %%ebp, %%ebp
-            \\ movq %%rsp, %[argc_argv_ptr]
+            \\ movq %%rsp, %%rdi
             \\ andq $-16, %%rsp
             \\ callq %[posixCallMainAndExit:P]
             ,
             .x86 =>
             \\ xorl %%ebp, %%ebp
-            \\ movl %%esp, %[argc_argv_ptr]
+            \\ movl %%esp, %%eax
             \\ andl $-16, %%esp
+            \\ subl $12, %%esp
+            \\ pushl %%eax
             \\ calll %[posixCallMainAndExit:P]
             ,
             .aarch64, .aarch64_be =>
             \\ mov fp, #0
             \\ mov lr, #0
             \\ mov x0, sp
-            \\ str x0, %[argc_argv_ptr]
             \\ b %[posixCallMainAndExit]
             ,
             .arm, .armeb, .thumb, .thumbeb =>
             \\ mov fp, #0
             \\ mov lr, #0
-            \\ str sp, %[argc_argv_ptr]
+            \\ mov a1, sp
             \\ and sp, #-16
             \\ b %[posixCallMainAndExit]
             ,
             .riscv64 =>
             \\ li s0, 0
             \\ li ra, 0
-            \\ sd sp, %[argc_argv_ptr]
+            \\ mv a0, sp
             \\ andi sp, sp, -16
             \\ tail %[posixCallMainAndExit]@plt
             ,
             .mips, .mipsel =>
             // The lr is already zeroed on entry, as specified by the ABI.
             \\ addiu $fp, $zero, 0
-            \\ sw $sp, %[argc_argv_ptr]
+            \\ move $a0, $sp
             \\ .set push
             \\ .set noat
             \\ addiu $1, $zero, -16
@@ -322,7 +321,7 @@ fn _start() callconv(.Naked) noreturn {
             .mips64, .mips64el =>
             // The lr is already zeroed on entry, as specified by the ABI.
             \\ addiu $fp, $zero, 0
-            \\ sd $sp, %[argc_argv_ptr]
+            \\ move $a0, $sp
             \\ .set push
             \\ .set noat
             \\ daddiu $1, $zero, -16
@@ -332,7 +331,7 @@ fn _start() callconv(.Naked) noreturn {
             ,
             .powerpc, .powerpcle =>
             // Setup the initial stack frame and clear the back chain pointer.
-            \\ stw 1, %[argc_argv_ptr]
+            \\ mr 3, 1
             \\ li 0, 0
             \\ stwu 1, -16(1)
             \\ stw 0, 0(1)
@@ -342,7 +341,7 @@ fn _start() callconv(.Naked) noreturn {
             .powerpc64, .powerpc64le =>
             // Setup the initial stack frame and clear the back chain pointer.
             // TODO: Support powerpc64 (big endian) on ELFv2.
-            \\ std 1, %[argc_argv_ptr]
+            \\ mr 3, 1
             \\ li 0, 0
             \\ stdu 0, -32(1)
             \\ mtlr 0
@@ -352,12 +351,12 @@ fn _start() callconv(.Naked) noreturn {
             // argc is stored after a register window (16 registers) plus stack bias
             \\ mov %%g0, %%i6
             \\ add %%o6, 2175, %%l0
-            \\ ba %[posixCallMainAndExit]
-            \\  stx %%l0, %[argc_argv_ptr]
+            \\ mov %%l0, %%o0
+            \\ ba,a %[posixCallMainAndExit]
             ,
             else => @compileError("unsupported arch"),
         }
-        : [argc_argv_ptr] "=m" (argc_argv_ptr),
+        :
         : [posixCallMainAndExit] "X" (&posixCallMainAndExit),
     );
 }
@@ -385,7 +384,7 @@ fn wWinMainCRTStartup() callconv(std.os.windows.WINAPI) noreturn {
     std.os.windows.ntdll.RtlExitUserProcess(@as(std.os.windows.UINT, @bitCast(result)));
 }
 
-fn posixCallMainAndExit() callconv(.C) noreturn {
+fn posixCallMainAndExit(argc_argv_ptr: [*]usize) callconv(.C) noreturn {
     const argc = argc_argv_ptr[0];
     const argv = @as([*][*:0]u8, @ptrCast(argc_argv_ptr + 1));