[libc-commits] [libc] [libc][thumb] support syscalls from thumb mode (PR #96558)

[Nick Desaulniers via libc-commits]

https://github.com/nickdesaulniers created https://github.com/llvm/llvm-project/pull/96558

r7 is reserved in thumb2 (typically for the frame pointer, as opposed to r11 in
ARM mode), so assigning to a variable with explicit register storage in r7 will
produce an error.

But r7 is where the Linux kernel expects the syscall number to be placed. We
can use a temporary to get the register allocator to pick a temporary, which we
save+restore the previous value of r7 in.

>From acb86604c1b775d46e9defd47ffaf1bf885fc571 Mon Sep 17 00:00:00 2001
From: Nick Desaulniers <ndesaulniers at google.com>
Date: Mon, 24 Jun 2024 13:45:07 -0700
Subject: [PATCH] [libc][thumb] support syscalls from thumb mode

r7 is reserved in thumb2 (typically for the frame pointer, as opposed to r11 in
ARM mode), so assigning to a variable with explicit register storage in r7 will
produce an error.

But r7 is where the Linux kernel expects the syscall number to be placed. We
can use a temporary to get the register allocator to pick a temporary, which we
save+restore the previous value of r7 in.
---
 libc/src/__support/OSUtil/linux/arm/syscall.h | 23 +++++++++++++------
 1 file changed, 16 insertions(+), 7 deletions(-)

diff --git a/libc/src/__support/OSUtil/linux/arm/syscall.h b/libc/src/__support/OSUtil/linux/arm/syscall.h
index af10074783275..926d857786eb2 100644
--- a/libc/src/__support/OSUtil/linux/arm/syscall.h
+++ b/libc/src/__support/OSUtil/linux/arm/syscall.h
@@ -12,14 +12,26 @@
 #include "src/__support/common.h"
 
 #ifdef __thumb__
-#error "The arm syscall implementation does not yet support thumb flavor."
-#endif // __thumb__
+#define R7 long r7 = number;
+#define SYSCALL_INSTR(input_constraint) \
+  int temp; \
+  LIBC_INLINE_ASM(R"(
+    mov %[temp], r7
+    mov r7, %2
+    svc #0
+    mov r7, %[temp]
+  )" : "=r"(r0), [temp] "=&r"(temp) : input_constraint : "memory", "cc")
+#else
+#define R7 register long r7 asm("r7") = number;
+#define SYSCALL_INSTR(input_constraint)                                        \
+  LIBC_INLINE_ASM("svc 0" : "=r"(r0) : input_constraint : "memory", "cc")
+#endif
 
 #define REGISTER_DECL_0                                                        \
-  register long r7 __asm__("r7") = number;                                     \
+  R7 \
   register long r0 __asm__("r0");
 #define REGISTER_DECL_1                                                        \
-  register long r7 __asm__("r7") = number;                                     \
+  R7 \
   register long r0 __asm__("r0") = arg1;
 #define REGISTER_DECL_2                                                        \
   REGISTER_DECL_1                                                              \
@@ -45,9 +57,6 @@
 #define REGISTER_CONSTRAINT_5 REGISTER_CONSTRAINT_4, "r"(r4)
 #define REGISTER_CONSTRAINT_6 REGISTER_CONSTRAINT_5, "r"(r5)
 
-#define SYSCALL_INSTR(input_constraint)                                        \
-  LIBC_INLINE_ASM("svc 0" : "=r"(r0) : input_constraint : "memory", "cc")
-
 namespace LIBC_NAMESPACE {
 
 LIBC_INLINE long syscall_impl(long number) {