[1/4] i386: Don't unnecessarily save and restore EAX, ECX and EDX [BZ# 25262]
Commit Message
On i386, since EAX, ECX and EDX are caller-saved, there are no need
to save and restore EAX, ECX and EDX in getcontext, setcontext and
swapcontext. They just need to clear EAX on success. The extra
scratch registers are needed to enable CET.
Tested on i386.
---
sysdeps/unix/sysv/linux/i386/getcontext.S | 8 +-------
sysdeps/unix/sysv/linux/i386/setcontext.S | 11 ++++-------
sysdeps/unix/sysv/linux/i386/swapcontext.S | 17 +++++------------
3 files changed, 10 insertions(+), 26 deletions(-)
@@ -26,13 +26,7 @@ ENTRY(__getcontext)
/* Load address of the context data structure. */
movl 4(%esp), %eax
- /* Return value of getcontext. EAX is the only register whose
- value is not preserved. */
- movl $0, oEAX(%eax)
-
- /* Save the 32-bit register values and the return address. */
- movl %ecx, oECX(%eax)
- movl %edx, oEDX(%eax)
+ /* Save the preserved register values and the return address. */
movl %edi, oEDI(%eax)
movl %esi, oESI(%eax)
movl %ebp, oEBP(%eax)
@@ -65,22 +65,19 @@ ENTRY(__setcontext)
cfi_offset (esi, oESI)
cfi_offset (ebp, oEBP)
cfi_offset (ebx, oEBX)
- cfi_offset (edx, oEDX)
- cfi_offset (ecx, oECX)
movl oESP(%eax), %esp
/* Push the return address on the new stack so we can return there. */
pushl %ecx
- /* Load the values of all the 32-bit registers (except ESP).
- Since we are loading from EAX, it must be last. */
+ /* Load the values of all the preserved registers (except ESP). */
movl oEDI(%eax), %edi
movl oESI(%eax), %esi
movl oEBP(%eax), %ebp
movl oEBX(%eax), %ebx
- movl oEDX(%eax), %edx
- movl oECX(%eax), %ecx
- movl oEAX(%eax), %eax
+
+ /* All done, return 0 for success. */
+ xorl %eax, %eax
/* End FDE here, we fall into another context. */
cfi_endproc
@@ -26,13 +26,7 @@ ENTRY(__swapcontext)
/* Load address of the context data structure we save in. */
movl 4(%esp), %eax
- /* Return value of swapcontext. EAX is the only register whose
- value is not preserved. */
- movl $0, oEAX(%eax)
-
- /* Save the 32-bit register values and the return address. */
- movl %ecx, oECX(%eax)
- movl %edx, oEDX(%eax)
+ /* Save the preserved register values and the return address. */
movl %edi, oEDI(%eax)
movl %esi, oESI(%eax)
movl %ebp, oEBP(%eax)
@@ -91,15 +85,14 @@ ENTRY(__swapcontext)
/* Push the return address on the new stack so we can return there. */
pushl %ecx
- /* Load the values of all the 32-bit registers (except ESP).
- Since we are loading from EAX, it must be last. */
+ /* Load the values of all the preserved registers (except ESP). */
movl oEDI(%eax), %edi
movl oESI(%eax), %esi
movl oEBP(%eax), %ebp
movl oEBX(%eax), %ebx
- movl oEDX(%eax), %edx
- movl oECX(%eax), %ecx
- movl oEAX(%eax), %eax
+
+ /* All done, return 0 for success. */
+ xorl %eax, %eax
/* The following 'ret' will pop the address of the code and jump
to it. */