qbe/ifopt.c
Roland Paterson-Jones 5c1eb24e2c If-conversion RFC 4 - x86 only (for now), use cmovXX
Replacement of tiny conditional jump graphlets with
conditional move instructions.

Currently enabled only for x86. Arm64 support using cselXX
will be essentially identical.

Adds (internal) frontend sel0/sel1 ops with flag-specific
backend xselXX following jnz implementation pattern.

Testing: standard QBE, cproc, harec, hare, roland
2026-01-13 18:11:30 +01:00

122 lines
2.1 KiB
C

#include "all.h"
enum {
MaxIns = 2,
MaxPhis = 2,
};
static int
okbranch(Blk *b)
{
Ins *i;
int n;
n = 0;
for (i=b->ins; i<&b->ins[b->nins]; i++)
if (i->op != Odbgloc) {
if (pinned(i))
return 0;
if (i->op != Onop)
n++;
}
return n <= MaxIns;
}
static int
okjoin(Blk *b)
{
Phi *p;
int n;
n = 0;
for (p=b->phi; p; p=p->link) {
if (KBASE(p->cls) != 0)
return 0;
n++;
}
return n <= MaxPhis;
}
static int
okgraph(Blk *ifb, Blk *thenb, Blk *elseb, Blk *joinb)
{
if (joinb->npred != 2 || !okjoin(joinb))
return 0;
assert(thenb != elseb);
if (thenb != ifb && !okbranch(thenb))
return 0;
if (elseb != ifb && !okbranch(elseb))
return 0;
return 1;
}
static void
convert(Blk *ifb, Blk *thenb, Blk *elseb, Blk *joinb)
{
Ins *ins, sel;
Phi *p;
uint nins;
ins = vnew(0, sizeof ins[0], PHeap);
nins = 0;
addbins(&ins, &nins, ifb);
if (thenb != ifb)
addbins(&ins, &nins, thenb);
if (elseb != ifb)
addbins(&ins, &nins, elseb);
assert(joinb->npred == 2);
if (joinb->phi) {
sel = (Ins){
.op = Osel0, .cls = Kw,
.arg = {ifb->jmp.arg},
};
addins(&ins, &nins, &sel);
}
sel = (Ins){.op = Osel1};
for (p=joinb->phi; p; p=p->link) {
sel.to = p->to;
sel.cls = p->cls;
sel.arg[0] = phiarg(p, thenb);
sel.arg[1] = phiarg(p, elseb);
addins(&ins, &nins, &sel);
}
idup(ifb, ins, nins);
ifb->jmp.type = Jjmp;
ifb->jmp.arg = R;
ifb->s1 = joinb;
ifb->s2 = 0;
joinb->npred = 1;
joinb->pred[0] = ifb;
joinb->phi = 0;
vfree(ins);
}
/* eliminate if-then[-else] graphlets
* using sel instructions
* needs rpo pred use; breaks cfg use
*/
void
ifconvert(Fn *fn)
{
Blk *ifb, *thenb, *elseb, *joinb;
if (debug['K'])
fputs("\n> If-conversion:\n", stderr);
for (ifb=fn->start; ifb; ifb=ifb->link)
if (ifgraph(ifb, &thenb, &elseb, &joinb))
if (okgraph(ifb, thenb, elseb, joinb)) {
if (debug['K'])
fprintf(stderr,
" @%s -> @%s, @%s -> @%s\n",
ifb->name, thenb->name, elseb->name,
joinb->name);
convert(ifb, thenb, elseb, joinb);
}
if (debug['K']) {
fprintf(stderr, "\n> After if-conversion:\n");
printfn(fn, stderr);
}
}