x86-64/swapcontext: Restore the pointer into %rdx after syscall

To prepare for shadow stack support, restore the pointer into %rdx after
syscall and use %rdx, instead of %rsi, to restore context.  There is no
functional change.

Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Reviewed-by: Carlos O'Donell <carlos@redhat.com>
	* sysdeps/unix/sysv/linux/x86_64/swapcontext.S (__swapcontext):
	Restore the pointer into %rdx, after syscall and use %rdx,
	instead of %rsi, to restore context.

diff --git a/sysdeps/unix/sysv/linux/x86_64/swapcontext.S b/sysdeps/unix/sysv/linux/x86_64/swapcontext.S
index e577c209b965a19be3be8f2abaeec9debd383da5..1110c479fad27829f1b1e480cf7520cf77d795f7 100644 (file)
--- a/sysdeps/unix/sysv/linux/x86_64/swapcontext.S
+++ b/sysdeps/unix/sysv/linux/x86_64/swapcontext.S
@@ -79,38 +79,40 @@ ENTRY(__swapcontext)
        cmpq    $-4095, %rax            /* Check %rax for error.  */
        jae     SYSCALL_ERROR_LABEL     /* Jump to error handler if error.  */
 
-       /* Restore destroyed registers.  */
-       movq    %r12, %rsi
+       /* Restore destroyed register into RDX. The choice is arbitrary,
+          but leaving RDI and RSI available for use later can avoid
+          shuffling values.  */
+       movq    %r12, %rdx
 
        /* Restore the floating-point context.  Not the registers, only the
           rest.  */
-       movq    oFPREGS(%rsi), %rcx
+       movq    oFPREGS(%rdx), %rcx
        fldenv  (%rcx)
-       ldmxcsr oMXCSR(%rsi)
+       ldmxcsr oMXCSR(%rdx)
 
        /* Load the new stack pointer and the preserved registers.  */
-       movq    oRSP(%rsi), %rsp
-       movq    oRBX(%rsi), %rbx
-       movq    oRBP(%rsi), %rbp
-       movq    oR12(%rsi), %r12
-       movq    oR13(%rsi), %r13
-       movq    oR14(%rsi), %r14
-       movq    oR15(%rsi), %r15
+       movq    oRSP(%rdx), %rsp
+       movq    oRBX(%rdx), %rbx
+       movq    oRBP(%rdx), %rbp
+       movq    oR12(%rdx), %r12
+       movq    oR13(%rdx), %r13
+       movq    oR14(%rdx), %r14
+       movq    oR15(%rdx), %r15
 
        /* The following ret should return to the address set with
        getcontext.  Therefore push the address on the stack.  */
-       movq    oRIP(%rsi), %rcx
+       movq    oRIP(%rdx), %rcx
        pushq   %rcx
 
        /* Setup registers used for passing args.  */
-       movq    oRDI(%rsi), %rdi
-       movq    oRDX(%rsi), %rdx
-       movq    oRCX(%rsi), %rcx
-       movq    oR8(%rsi), %r8
-       movq    oR9(%rsi), %r9
-
-       /* Setup finally  %rsi.  */
-       movq    oRSI(%rsi), %rsi
+       movq    oRDI(%rdx), %rdi
+       movq    oRSI(%rdx), %rsi
+       movq    oRCX(%rdx), %rcx
+       movq    oR8(%rdx), %r8
+       movq    oR9(%rdx), %r9
+
+       /* Setup finally %rdx.  */
+       movq    oRDX(%rdx), %rdx
 
        /* Clear rax to indicate success.  */
        xorl    %eax, %eax