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

1 files changed, 45 insertions, 0 deletions

diff --git a/cfg.c b/cfg.c
index 8047c12..8d31f18 100644
--- a/cfg.c
+++ b/cfg.c
@@ -396,3 +396,48 @@ reachesnotvia(Fn *fn, Blk *b, Blk *to, Blk *excl)
 	excl->visit = 1;
 	return reaches(fn, b, to);
 }
+
+int
+ifgraph(Blk *ifb, Blk **pthenb, Blk **pelseb, Blk **pjoinb)
+{
+	Blk *s1, *s2, **t;
+
+	if (ifb->jmp.type != Jjnz)
+		return 0;
+
+	s1 = ifb->s1;
+	s2 = ifb->s2;
+	if (s1->id > s2->id) {
+		s1 = ifb->s2;
+		s2 = ifb->s1;
+		t = pthenb;
+		pthenb = pelseb;
+		pelseb = t;
+	}
+	if (s1 == s2)
+		return 0;
+
+	if (s1->jmp.type != Jjmp || s1->npred != 1)
+		return 0;
+
+	if (s1->s1 == s2) {
+		/* if-then / if-else */
+		if (s2->npred != 2)
+			return 0;
+		*pthenb = s1;
+		*pelseb = ifb;
+		*pjoinb = s2;
+		return 1;
+	}
+
+	if (s2->jmp.type != Jjmp || s2->npred != 1)
+		return 0;
+	if (s1->s1 != s2->s1 || s1->s1->npred != 2)
+		return 0;
+
+	assert(s1->s1 != ifb);
+	*pthenb = s1;
+	*pelseb = s2;
+	*pjoinb = s1->s1;
+	return 1;
+}