[compiler-rt] Mark $t* as clobbered for Linux/LoongArch syscalls

[lixing-star]

Summary

Linux/LoongArch doesn't preserve temporary registers across syscalls,
so we have to explicitly mark them as clobbered to avoid trashing local variables.

[xry111]

No, a1-a7 are saved by syscall.

Link: https://lore.kernel.org/loongarch/[e-mail]/T/#m4964e2aad6c5ee28ccd51d4b22f56d46f93fc369

[xry111]

compiler-rt/lib/sanitizer_common/sanitizer_syscall_linux_loongarch64.inc

25

Only this part is needed.

[lixing-star]

According to the ABI document, only t0~t8 need to be added to the clobber field

[xry111]

Thanks for fixing another instance of stupid programming errors made by me!

[xen0n]

Edited the patch summary for you so it's more concise. The code changes look good. Thanks!

[SixWeining]

LGTM.
But I see the comments in this file:

// Kernel ABI...
//  syscall number is passed in a7
//  (http://man7.org/linux/man-pages/man2/syscall.2.html) results are return in
//  a0 and a1 (http://man7.org/linux/man-pages/man2/syscall.2.html) arguments
//  are passed in: a0-a7 (confirmed by inspecting glibc sources).

// results are return in a0 and a1

Is this correct?

https://lore.kernel.org/loongarch/[e-mail]/T/#m4964e2aad6c5ee28ccd51d4b22f56d46f93fc369 says:

Upon return, `a0` contains the return value

[xry111]

In D137396#3909664, @SixWeining wrote:

LGTM.
But I see the comments in this file:

// Kernel ABI...
//  syscall number is passed in a7
//  (http://man7.org/linux/man-pages/man2/syscall.2.html) results are return in
//  a0 and a1 (http://man7.org/linux/man-pages/man2/syscall.2.html) arguments
//  are passed in: a0-a7 (confirmed by inspecting glibc sources).

// results are return in a0 and a1

Is this correct?

No. Another of my stupid errors, caused by copying from RISC-V without a close inspection.

[xry111]

-// Kernel ABI...
-//  syscall number is passed in a7
-//  (http://man7.org/linux/man-pages/man2/syscall.2.html) results are return in
-//  a0 and a1 (http://man7.org/linux/man-pages/man2/syscall.2.html) arguments
-//  are passed in: a0-a7 (confirmed by inspecting glibc sources).
+// Kernel ABI:
+// https://lore.kernel.org/loongarch/[e-mail]/T/#m1613bc86c2d7bf5f6da92bd62984302bfd699a2f
+//  syscall number is placed in a7
+//  parameters, if present, are placed in a0-a6
+//  upon return:
+//    the return value is placed in a0
+//    t0-t8 should be considered clobbered
+//    all other registers are preserved

Is it possible to add this part into this diff? If not I'll create a new diff.

[lixing-star]

In D137396#3909672, @xry111 wrote:

-// Kernel ABI...
-//  syscall number is passed in a7
-//  (http://man7.org/linux/man-pages/man2/syscall.2.html) results are return in
-//  a0 and a1 (http://man7.org/linux/man-pages/man2/syscall.2.html) arguments
-//  are passed in: a0-a7 (confirmed by inspecting glibc sources).
+// Kernel ABI:
+// https://lore.kernel.org/loongarch/[e-mail]/T/#m1613bc86c2d7bf5f6da92bd62984302bfd699a2f
+//  syscall number is placed in a7
+//  parameters, if present, are placed in a0-a6
+//  upon return:
+//    the return value is placed in a0
+//    t0-t8 should be considered clobbered
+//    all other registers are preserved

Is it possible to add this part into this diff? If not I'll create a new diff.

ok, I will change the comment and submit again

compiler-rt/lib/sanitizer_common/sanitizer_syscall_linux_loongarch64.inc

25

yes, you are right, the handle_syscall saved a0~a7 already

compiler-rt/lib/sanitizer_common/sanitizer_syscall_linux_loongarch64.inc

 //===-- sanitizer_syscall_linux_loongarch64.inc -----------------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // Implementations of internal_syscall and internal_iserror for
 // Linux/loongarch64.
 //
 //===----------------------------------------------------------------------===//
 
 // About local register variables:
 // https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html#Local-Register-Variables
 //
-// Kernel ABI...
-//  syscall number is passed in a7
-//  (http://man7.org/linux/man-pages/man2/syscall.2.html) results are return in
-//  a0 and a1 (http://man7.org/linux/man-pages/man2/syscall.2.html) arguments
-//  are passed in: a0-a7 (confirmed by inspecting glibc sources).
+// Kernel ABI:
+// https://lore.kernel.org/loongarch/[e-mail]/T/#m1613bc86c2d7bf5f6da92bd62984302bfd699a2f
+//  syscall number is placed in a7
+//  parameters, if present, are placed in a0-a6
+//  upon return:
+//    the return value is placed in a0
+//    t0-t8 should be considered clobbered
+//    all other registers are preserved
 #define SYSCALL(name) __NR_##name
 
-#define INTERNAL_SYSCALL_CLOBBERS "memory"
+#define INTERNAL_SYSCALL_CLOBBERS \
+  "memory", "$t0", "$t1", "$t2", "$t3", "$t4", "$t5", "$t6", "$t7", "$t8"
 
 static uptr __internal_syscall(u64 nr) {
   register u64 a7 asm("$a7") = nr;
   register u64 a0 asm("$a0");
   __asm__ volatile("syscall 0\n\t"
                    : "=r"(a0)
                    : "r"(a7)
                    : INTERNAL_SYSCALL_CLOBBERS);