morestack.S: Correct CFI information to do proper returns throughout function.

	* config/i386/morestack.S: Correct CFI information to do proper
	returns throughout function.  In 32-bit mode, save %ebx so that it
	is restored on unwind.

From-SVN: r180633

libgcc/ChangeLog

+2011-10-28  Ian Lance Taylor  <iant@google.com>
+
+	* config/i386/morestack.S: Correct CFI information to do proper
+	returns throughout function.  In 32-bit mode, save %ebx so that it
+	is restored on unwind.
+
 2011-10-25  Bernd Schmidt  <bernds@codesourcery.com>
 
 	* config/c6x/pr-support.c (__gnu_unwind_24bit): Correct logic for the

libgcc/config/i386/morestack.S

@@ -139,44 +139,68 @@ __morestack:
 	.cfi_lsda 0x1b,.LLSDA1
 #endif
 
-	# Set up a normal backtrace.
-	pushl	%ebp
-	.cfi_def_cfa_offset 8
-	.cfi_offset %ebp, -8
-	movl	%esp, %ebp
-	.cfi_def_cfa_register %ebp
-
 	# We return below with a ret $8.  We will return to a single
 	# return instruction, which will return to the caller of our
 	# caller.  We let the unwinder skip that single return
 	# instruction, and just return to the real caller.
-	.cfi_offset 8, 8
+
+	# Here CFA points just past the return address on the stack,
+	# e.g., on function entry it is %esp + 4.  Later we will
+	# change it to %ebp + 8, as set by .cfi_def_cfa_register and
+	# .cfi_def_cfa_offset above.  The stack looks like this:
+	#	CFA + 12:	stack pointer after two returns
+	#	CFA + 8:	return address of morestack caller's caller
+	#	CFA + 4:	size of parameters
+	#	CFA:		new stack frame size
+	#	CFA - 4:	return address of this function
+	#	CFA - 8:	previous value of %ebp; %ebp points here
+	# We want to set %esp to the stack pointer after the double
+	# return, which is CFA + 12.
+	.cfi_offset 8, 8		# New PC stored at CFA + 8
 	.cfi_escape 0x15, 4, 0x7d	# DW_CFA_val_offset_sf, %esp, 12/-4
+					# i.e., next %esp is CFA + 12
+
+	# Set up a normal backtrace.
+	pushl	%ebp
+	.cfi_def_cfa_offset 8
+	.cfi_offset %ebp, -8
+	movl	%esp,%ebp
+	.cfi_def_cfa_register %ebp
 
 	# In 32-bit mode the parameters are pushed on the stack.  The
 	# argument size is pushed then the new stack frame size is
 	# pushed.
 
+	# Align stack to 16-byte boundary with enough space for saving
+	# registers and passing parameters to functions we call.
+	subl	$40,%esp
+
+	# Because our cleanup code may need to clobber %ebx, we need
+	# to save it here so the unwinder can restore the value used
+	# by the caller.  Note that we don't have to restore the
+	# register, since we don't change it, we just have to save it
+	# for the unwinder.
+	movl	%ebx,-4(%ebp)
+	.cfi_offset %ebx, -12
+
 	# In 32-bit mode the registers %eax, %edx, and %ecx may be
 	# used for parameters, depending on the regparm and fastcall
 	# attributes.
 
-	pushl	%eax
-	pushl	%edx
-	pushl	%ecx
+	movl	%eax,-8(%ebp)
+	movl	%edx,-12(%ebp)
+	movl	%ecx,-16(%ebp)
 
 	call	__morestack_block_signals
 
-	pushl	12(%ebp)		# The size of the parameters.
+	movl	12(%ebp),%eax		# The size of the parameters.
+	movl	%eax,8(%esp)
 	leal	20(%ebp),%eax		# Address of caller's parameters.
-	pushl	%eax
+	movl	%eax,4(%esp)
 	addl	$BACKOFF,8(%ebp)	# Ask for backoff bytes.
 	leal	8(%ebp),%eax		# The address of the new frame size.
-	pushl	%eax
+	movl	%eax,(%esp)
 
-	# Note that %esp is exactly 32 bytes below the CFA -- perfect for
-	# a 16-byte aligned stack.  That said, we still ought to compile
-	# generic-morestack.c with -mpreferred-stack-boundary=2.  FIXME.
 	call	__generic_morestack
 
 	movl	%eax,%esp		# Switch to the new stack.
@@ -191,8 +215,8 @@ __morestack:
 
 	call	__morestack_unblock_signals
 
-	movl	-8(%ebp),%edx		# Restore registers.
-	movl	-12(%ebp),%ecx
+	movl	-12(%ebp),%edx		# Restore registers.
+	movl	-16(%ebp),%ecx
 
 	movl	4(%ebp),%eax		# Increment the return address
 	cmpb	$0xc3,(%eax)		# to skip the ret instruction;
@@ -200,12 +224,12 @@ __morestack:
 	addl	$2,%eax
 1:	inc	%eax
 
-	movl	%eax,-8(%ebp)		# Store return address in an
+	movl	%eax,-12(%ebp)		# Store return address in an
 					# unused slot.
 
-	movl	-4(%ebp),%eax		# Restore the last register.
+	movl	-8(%ebp),%eax		# Restore the last register.
 
-	call	*-8(%ebp)		# Call our caller!
+	call	*-12(%ebp)		# Call our caller!
 
 	# The caller will return here, as predicted.
 
@@ -255,9 +279,13 @@ __morestack:
 	popl	%eax
 
 	.cfi_remember_state
+
+	# We never changed %ebx, so we don't have to actually restore it.
+	.cfi_restore %ebx
+
 	popl	%ebp
 	.cfi_restore %ebp
-	.cfi_def_cfa %esp, 12
+	.cfi_def_cfa %esp, 4
 	ret	$8			# Return to caller, which will
 					# immediately return.  Pop
 					# arguments as we go.
@@ -300,13 +328,6 @@ __morestack:
 	.cfi_lsda 0x1b,.LLSDA1
 #endif
 
-	# Set up a normal backtrace.
-	pushq	%rbp
-	.cfi_def_cfa_offset 16
-	.cfi_offset %rbp, -16
-	movq	%rsp, %rbp
-	.cfi_def_cfa_register %rbp
-
 	# We will return a single return instruction, which will
 	# return to the caller of our caller.  Let the unwinder skip
 	# that single return instruction, and just return to the real
@@ -314,6 +335,13 @@ __morestack:
 	.cfi_offset 16, 0
 	.cfi_escape 0x15, 7, 0x7f	# DW_CFA_val_offset_sf, %esp, 8/-8
 
+	# Set up a normal backtrace.
+	pushq	%rbp
+	.cfi_def_cfa_offset 16
+	.cfi_offset %rbp, -16
+	movq	%rsp, %rbp
+	.cfi_def_cfa_register %rbp
+
 	# In 64-bit mode the new stack frame size is passed in r10
         # and the argument size is passed in r11.