[PATCH 1/4] i386: Don't unnecessarily save and restore EAX, ECX and EDX [BZ# 25262]

["H.J. Lu" <hjl.tools@gmail.com>]

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(-)

diff --git a/sysdeps/unix/sysv/linux/i386/getcontext.S b/sysdeps/unix/sysv/linux/i386/getcontext.S
index 1568f9604a..85da8195f9 100644
--- a/sysdeps/unix/sysv/linux/i386/getcontext.S
+++ b/sysdeps/unix/sysv/linux/i386/getcontext.S
@@ -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)
diff --git a/sysdeps/unix/sysv/linux/i386/setcontext.S b/sysdeps/unix/sysv/linux/i386/setcontext.S
index dffe1d18e7..9095584b44 100644
--- a/sysdeps/unix/sysv/linux/i386/setcontext.S
+++ b/sysdeps/unix/sysv/linux/i386/setcontext.S
@@ -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
diff --git a/sysdeps/unix/sysv/linux/i386/swapcontext.S b/sysdeps/unix/sysv/linux/i386/swapcontext.S
index 6fdc176535..83e5f0b2ff 100644
--- a/sysdeps/unix/sysv/linux/i386/swapcontext.S
+++ b/sysdeps/unix/sysv/linux/i386/swapcontext.S
@@ -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.  */
-- 
2.21.0