[Bug target/30354] -Os doesn't optimize a/CONST even if it saves size.

[vda dot linux at googlemail dot com]

------- Comment #2 from vda dot linux at googlemail dot com  2007-07-25 15:05 -------
Created an attachment (id=13973)
 --> (http://gcc.gnu.org/bugzilla/attachment.cgi?id=13973&action=view)
Fix: adjust div cost for -Os on i386 

Patch was tested with 4.2.1, I guess it will apply to other versions of gcc as
it is quite trivial.

Test program with lots or randomly-generated constant divisions and %'s was
compiled by patched gcc with different costs of division:

   text    data     bss     dec     hex filename
 257731       0       0  257731   3eec3 421.org/t-Os.o
 256788       0       0  256788   3eb14 421.7/t-Os.o
 256788       0       0  256788   3eb14 421.8/t-Os.o
 257377       0       0  257377   3ed61 421.9/t-Os.o
 257825       0       0  257825   3ef21 421.10/t-Os.o

Seems like (at least on 4.2.1) cost of 8 is giving smallest code.

Among 15000 divisions in test program, only signed divisions by power-of-two
grew in size (but they become MUCH faster, as they don't even use multiply now,
let alone div):

@@ -1703 +1703 @@
-0000000f T id_x_16
+00000012 T id_x_16
@@ -1836 +1836 @@
-0000000f T id_x_2
+0000000e T id_x_2
@@ -2030 +2030 @@
-0000000f T id_x_32
+00000012 T id_x_32

id_x_16 was:
        movl    4(%esp), %eax
        movl    $16, %edx
        movl    %edx, %ecx
        cltd
        idivl   %ecx
        ret
Now it is:
        movl    4(%esp), %edx
        movl    %edx, %eax
        sarl    $31, %eax
        andl    $15, %eax
        addl    %edx, %eax
        sarl    $4, %eax
        ret

and also unsigned_x / 28, unsigned_x / 13952, unsigned_x / 56 grew by 1 byte.

The rest either were not changed (and still use div insn) or shrank (typically
by 1 byte, record holders are "unsigned_x / 641" and "unsigned_x / 6700417" -
shrank by 4 bytes).


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=30354