Re: [Tinycc-devel] Re : Re: Re : Some questions regarding of TCC's optimizations.

[Brian Callahan]

So then let's take it slow, starting with the lowest of low-hanging
fruit and showing numbers along the way.

The patch below improves the x86_64 codegen to convert:
movl $0, %e{ax,cx,dx,sp,si,di}
to:
xorl %e{ax,cx,dx,sp,si,di}, %e{ax,cx,dx,sp,si,di}

Here are benchmark numbers of the .text sizes of a 3-way bootstrap tcc,
before this patch (old) and after this patch (new):

bin             old     new     diff    %reduction
---             ---     ---     ----    ----------
tcc             328786  325878  2908    0.88
libtcc.a        20214   20213   201     0.99
bcheck.o        23254   23209   45      0.19
bt-exe.o        4732    4693    39      0.82
bt-log.o        648     648     0       0
libtcc1.a       12678   12498   180     1.42

So it's not much, but it's also not nothing (except for the bt-log.o
case, where it literally is nothing).

And timing results for the compiling tcc.c 10 times test:

old             new
---             ---
1010 ms         1008 ms

...which is just a fancy way of saying there appears to be no
statistical difference in speed after applying this patch. All these
measurements were run on my machine, which is an OpenBSD/amd64 machine
with a very old and slow i3 CPU.

This patch could easily be applied to movq $0, %r{ax,cx,dx,sp,si,di},
which I'll send in a follow-up email. The %r8-%15 and equivalent
%r8d-%r15d conversions are slightly different. The i386 codegen should
be able to have a mechanical translation of this diff applied to it as well.

~Brian

diff --git a/x86_64-gen.c b/x86_64-gen.c
index 81ec5d9..775e132 100644
--- a/x86_64-gen.c
+++ b/x86_64-gen.c
@@ -486,8 +486,13 @@ void load(int r, SValue *sv)
                 orex(1,r,0, 0xb8 + REG_VALUE(r)); /* mov $xx, r */
                 gen_le64(sv->c.i);
             } else {
-                orex(0,r,0, 0xb8 + REG_VALUE(r)); /* mov $xx, r */
-                gen_le32(fc);
+                if (fc == 0 && r < 8) {
+                    o(0x31); /* xor r, r */
+                    o(0xc0 + REG_VALUE(r) * 9);
+                } else {
+                    orex(0,r,0, 0xb8 + REG_VALUE(r)); /* mov $xx, r */
+                    gen_le32(fc);
+                }
             }
         } else if (v == VT_LOCAL) {
             orex(1,0,r,0x8d); /* lea xxx(%ebp), r */