i386.c (override_options): Default to minimum stack alignment when optimizing for code size.

* config/i386/i386.c (override_options): Default to minimum
stack alignment when optimizing for code size.
* doc/invoke.texi (-mpreferred-stack-boundary): Document the
change.

From-SVN: r46433

3 files changed, 21 insertions, 11 deletions

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 8d2b667792a..4db00904ef4 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,5 +1,10 @@
 2001-10-23  Alexandre Oliva  <aoliva@redhat.com>
 
+	* config/i386/i386.c (override_options): Default to minimum
+	stack alignment when optimizing for code size.
+	* doc/invoke.texi (-mpreferred-stack-boundary): Document the
+	change.
+
 	* config/i386/i386.c (override_options): Set upper limit of
 	-mpreferred-stack-boundary to 12.
 
diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c
index 5183c5f5e11..06dc3be823e 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -973,8 +973,12 @@ override_options ()
     align_functions = 1 << abs (processor_target_table[ix86_cpu].align_func);
 
   /* Validate -mpreferred-stack-boundary= value, or provide default.
-     The default of 128 bits is for Pentium III's SSE __m128.  */
-  ix86_preferred_stack_boundary = 128;
+     The default of 128 bits is for Pentium III's SSE __m128, but we
+     don't want additional code to keep the stack aligned when
+     optimizing for code size.  */
+  ix86_preferred_stack_boundary = (optimize_size
+				   ? TARGET_64BIT ? 64 : 32
+				   : 128);
   if (ix86_preferred_stack_boundary_string)
     {
       i = atoi (ix86_preferred_stack_boundary_string);
diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index dc1c58feb48..1054c2bf358 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -7587,12 +7587,13 @@ startup modules.
 @opindex mpreferred-stack-boundary
 Attempt to keep the stack boundary aligned to a 2 raised to @var{num}
 byte boundary.  If @option{-mpreferred-stack-boundary} is not specified,
-the default is 4 (16 bytes or 128 bits).
+the default is 4 (16 bytes or 128 bits), except when optimizing for code
+size (@option{-Os}), in which case the default is the minimum correct
+alignment (4 bytes for x86, and 8 bytes for x86-64).
 
-The stack is required to be aligned on a 4 byte boundary.  On Pentium
-and PentiumPro, @code{double} and @code{long double} values should be
-aligned to an 8 byte boundary (see @option{-malign-double}) or suffer
-significant run time performance penalties.  On Pentium III, the
+On Pentium and PentiumPro, @code{double} and @code{long double} values
+should be aligned to an 8 byte boundary (see @option{-malign-double}) or
+suffer significant run time performance penalties.  On Pentium III, the
 Streaming SIMD Extension (SSE) data type @code{__m128} suffers similar
 penalties if it is not 16 byte aligned.
 
@@ -7604,10 +7605,10 @@ stack boundary from a function compiled with a lower preferred stack
 boundary will most likely misalign the stack.  It is recommended that
 libraries that use callbacks always use the default setting.
 
-This extra alignment does consume extra stack space.  Code that is sensitive
-to stack space usage, such as embedded systems and operating system kernels,
-may want to reduce the preferred alignment to
-@option{-mpreferred-stack-boundary=2}.
+This extra alignment does consume extra stack space, and generally
+increases code size.  Code that is sensitive to stack space usage, such
+as embedded systems and operating system kernels, may want to reduce the
+preferred alignment to @option{-mpreferred-stack-boundary=2}.
 
 @item -mmmx
 @itemx -mno-mmx