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Subject[PATCH] tools/nolibc: x86: Remove `r8`, `r9` and `r10` from the clobber list
Date
Linux x86-64 syscall only clobbers rax, rcx and r11 (and "memory").

- rax for the return value.
- rcx to save the return address.
- r11 to save the rflags.

Other registers are preserved.

Having r8, r9 and r10 in the syscall clobber list is harmless, but this
results in a missed-optimization.

As the syscall doesn't clobber r8-r10, GCC should be allowed to reuse
their value after the syscall returns to userspace. But since they are
in the clobber list, GCC will always miss this opportunity.

Remove them from the x86-64 syscall clobber list to help GCC generate
better code and fix the comment.

Cc: Andy Lutomirski <luto@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: x86@kernel.org
Cc: "H. Peter Anvin" <hpa@zytor.com>
Signed-off-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
---
tools/include/nolibc/nolibc.h | 28 ++++++++++++++--------------
1 file changed, 14 insertions(+), 14 deletions(-)

diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h
index 3430667b0d24..8c9f2202d6b6 100644
--- a/tools/include/nolibc/nolibc.h
+++ b/tools/include/nolibc/nolibc.h
@@ -265,7 +265,7 @@ struct stat {
* - arguments are in rdi, rsi, rdx, r10, r8, r9 respectively
* - the system call is performed by calling the syscall instruction
* - syscall return comes in rax
- * - rcx and r8..r11 may be clobbered, others are preserved.
+ * - rcx and r11 are clobbered, others are preserved.
* - the arguments are cast to long and assigned into the target registers
* which are then simply passed as registers to the asm code, so that we
* don't have to experience issues with register constraints.
@@ -280,9 +280,9 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret) \
+ : "=a"(_ret) \
: "0"(_num) \
- : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \
+ : "rcx", "r11", "memory", "cc" \
); \
_ret; \
})
@@ -295,10 +295,10 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret) \
+ : "=a"(_ret) \
: "r"(_arg1), \
"0"(_num) \
- : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \
+ : "rcx", "r11", "memory", "cc" \
); \
_ret; \
})
@@ -312,10 +312,10 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret) \
+ : "=a"(_ret) \
: "r"(_arg1), "r"(_arg2), \
"0"(_num) \
- : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \
+ : "rcx", "r11", "memory", "cc" \
); \
_ret; \
})
@@ -330,10 +330,10 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret) \
+ : "=a"(_ret) \
: "r"(_arg1), "r"(_arg2), "r"(_arg3), \
"0"(_num) \
- : "rcx", "r8", "r9", "r10", "r11", "memory", "cc" \
+ : "rcx", "r11", "memory", "cc" \
); \
_ret; \
})
@@ -349,10 +349,10 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret), "=r"(_arg4) \
+ : "=a"(_ret) \
: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), \
"0"(_num) \
- : "rcx", "r8", "r9", "r11", "memory", "cc" \
+ : "rcx", "r11", "memory", "cc" \
); \
_ret; \
})
@@ -369,10 +369,10 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret), "=r"(_arg4), "=r"(_arg5) \
+ : "=a" (_ret) \
: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
"0"(_num) \
- : "rcx", "r9", "r11", "memory", "cc" \
+ : "rcx", "r11", "memory", "cc" \
); \
_ret; \
})
@@ -390,7 +390,7 @@ struct stat {
\
asm volatile ( \
"syscall\n" \
- : "=a" (_ret), "=r"(_arg4), "=r"(_arg5) \
+ : "=a"(_ret) \
: "r"(_arg1), "r"(_arg2), "r"(_arg3), "r"(_arg4), "r"(_arg5), \
"r"(_arg6), "0"(_num) \
: "rcx", "r11", "memory", "cc" \
--
2.30.2
\
 
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