i found a simple solution now:
diff -r 46d745fbf479 sys/src/cmd/6c/mul.c
--- a/sys/src/cmd/6c/mul.c Thu Mar 20 16:44:01 2014 +0100
+++ b/sys/src/cmd/6c/mul.c Fri Mar 21 00:03:49 2014 +0100
@@ -312,6 +312,11 @@
Mparam *p;
Node nod, nods;
+ if(v == 0){
+ zeroregm(n);
+ return 1;
+ }
+
for(i = 0; i < nelem(multab); i++) {
p = &multab[i];
if(p->value == v)
as expected, the code generated *without* optimization does
some stupid moves:
term% ./6.out -SN /tmp/a.c
TEXT bar+0(SB),0,$0
MOVL BP,a+0(FP)
MOVL a+0(FP),AX
MOVL $0,AX
MOVL AX,a+0(FP)
MOVL a+0(FP),AX
NOP ,X0
RET ,
NOP ,AX
NOP ,X0
RET ,
TEXT foo+0(SB),0,$16
MOVL BP,a+0(FP)
MOVL a+0(FP),BP
CALL ,bar+0(SB)
MOVL $0,AX
NOP ,X0
RET ,
NOP ,AX
NOP ,X0
RET ,
TEXT main+0(SB),0,$16
MOVL BP,argc+0(FP)
MOVL $1,BP
CALL ,foo+0(SB)
NOP ,AX
NOP ,X0
RET ,
END ,
with optimization, the stupid moves are gone:
term% ./6.out -S /tmp/a.c
TEXT bar+0(SB),0,$0
MOVL $0,AX
RET ,
RET ,
TEXT foo+0(SB),0,$16
CALL ,bar+0(SB)
MOVL $0,AX
RET ,
RET ,
TEXT main+0(SB),0,$16
MOVL $1,BP
CALL ,foo+0(SB)
RET ,
END ,
so this is as good as the initial attempt just that
the change is just 4 lines in one location where we
do the actual multiplication.
--
cinap
