On Jun 25, 2008, at 11:17 AM, Scott Wood wrote:
Gabriel Paubert wrote:
On Wed, Jun 25, 2008 at 10:34:32AM -0500, Scott Wood wrote:
Kumar Gala wrote:
+/* Macros to workout the correct index for the FPR in the
thread struct */
+#define FPRNUMBER(i) (((i) - PT_FPR0) >> 1)
+#define FPRHALF(i) (((i) - PT_FPR0) % 2)
Have you looked at what the compiler spits out here to make sure
we aren't getting a divide? Seems like we could use '& 0x1'.
GCC's not *that* dumb. However, you may get some unnecessary sign-
twiddling if "i" is signed.
Not for modulo 2, it's only an even/odd choice and GCC implements
that efficiently IIRC. For other powers of 2,
making the left hand side unsigned helps the compiler.
From this:
int foo(int x)
{
return x % 2;
}
I get this with -O3:
foo:
mr 0,3
srawi 3,3,1
addze 3,3
slwi 3,3,1
subf 3,3,0
blr
.size foo, .-foo
.ident "GCC: (GNU) 4.1.2"
Changing it to "x & 1", or to unsigned, gives this:
foo:
rlwinm 3,3,0,31,31
blr
.size foo, .-foo
.ident "GCC: (GNU) 4.1.2"
Maybe newer GCCs are better?
Nope. gcc-4.3.0 from fedora 9:
foo:
mr 0,3
srawi 3,3,1
addze 3,3
slwi 3,3,1
subf 3,3,0
blr
bar:
rlwinm 3,3,0,31,31
blr
if you make 'x' unsigned things are better.
- k
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