On Sat, 6 May 2000, Dan Nelson wrote:
> In the last episode (May 05), Jean-Marc Zucconi said:
> > Here is something I don't understand:
> >
> > $ sh -c '/usr/bin/time ./a.out'
> > 2.40 real 2.38 user 0.01 sys
> > $ /usr/bin/time ./a.out
> > 7.19 real 7.19 user 0.00 sys
> It has to do with your stack. Calling the program via /bin/sh sets up
> your environment differently, so your program's stack starts at a
> different place. Try running this:
> Here are some bits from the gcc infopage explaining your options if you
> want consistant speed from programs using doubles:
>
> `-mpreferred-stack-boundary=NUM'
> Attempt to keep the stack boundary aligned to a 2 raised to NUM
> byte boundary. If `-mpreferred-stack-boundary' is not specified,
> the default is 4 (16 bytes or 128 bits).
> The stack is required to be aligned on a 4 byte boundary. On
> Pentium and PentiumPro, `double' and `long double' values should be
> aligned to an 8 byte boundary (see `-malign-double') or suffer
> significant run time performance penalties. On Pentium III, the
> Streaming SIMD Extention (SSE) data type `__m128' suffers similar
> penalties if it is not 16 byte aligned.
The default of 4 for -mpreferred-stack-boundary perfectly preserves
any initial misaligment of the stack. Under FreeBSD the stack is
initially misaligned (for doubles) with a probability of 1/2. There
was some discussion of fixing this when gcc-2.95 was imported, but
nothing was committed. I use the following local hack:
diff -c2 kern_exec.c~ kern_exec.c
*** kern_exec.c~ Mon May 1 15:56:40 2000
--- kern_exec.c Mon May 1 15:56:42 2000
***************
*** 627,630 ****
--- 647,659 ----
vectp = (char **)
(destp - (imgp->argc + imgp->envc + 2) * sizeof(char*));
+
+ /*
+ * Align stack to a multiple of 0x20.
+ * XXX vectp has the wrong type; we usually want a vm_offset_t;
+ * the suword() family takes a void *, but should take a vm_offset_t.
+ * XXX should align stack for signals too.
+ * XXX should do this more machine/compiler-independently.
+ */
+ vectp = (char **)(((vm_offset_t)vectp & ~(vm_offset_t)0x1F) - 4);
/*
Bruce
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