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.
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);
@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.
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