ir: simplify some occurrences of single-argument phis

2 files changed, 17 insertions, 8 deletions

diff --git a/ir/builder.c b/ir/builder.c
index c793f80..4b25329 100644
--- a/ir/builder.c
+++ b/ir/builder.c
@@ -228,19 +228,18 @@ useblk(struct function *fn, struct block *blk)
 union ref
 addphi(struct function *fn, enum irclass cls, union ref *r)
 {
-   int new;
-   struct instr ins = { Ophi, cls };
-   union ref *refs = NULL;
+   assert(fn->curblk);
+   if (fn->curblk->npred == 0) return UNDREF;
+   if (fn->curblk->npred == 1) /* 1-argument phi is identity */
+      return *r;
 
+   union ref *refs = NULL;
    xbgrow(&refs, fn->curblk->npred);
    memcpy(refs, r, fn->curblk->npred * sizeof *r);
    vpush(&phitab, refs);
-   ins.l = mkref(RXXX, phitab.n-1);
-
-   assert(fn->curblk != NULL);
    /*assert(fn->curblk->ins.n == 0);*/
-   new = allocinstr();
-   instrtab[new] = ins;
+   int new = allocinstr();
+   instrtab[new] = mkinstr(Ophi, cls, .l.i = phitab.n-1);
    for (int i = 0; i < fn->curblk->npred; ++i) {
       adduse(fn->curblk, new, r[i]);
    }
diff --git a/ir/ssa.c b/ir/ssa.c
index f6c9d6f..5c9c8f3 100644
--- a/ir/ssa.c
+++ b/ir/ssa.c
@@ -7,6 +7,16 @@ copyopt(struct function *fn)
 
    FREQUIRE(FNUSE);
    do {
+      if (blk->npred == 1) for (int i = 0; i < blk->phi.n; ++i) {
+         /* simplify 1-arg phi */
+         int phi = blk->phi.p[i];
+         union ref *arg = phitab.p[instrtab[phi].l.i];
+         /* being conservative here because phis could have circular dependencies? */
+         if (arg->t != RTMP || instrtab[arg->i].op != Ophi) {
+            replcuses(mkref(RTMP, phi), *arg);
+            delphi(blk, i--);
+         }
+      }
       for (int i = 0; i < blk->ins.n; ++i) {
          union ref var = mkref(RTMP, blk->ins.p[i]);
          struct instr *ins = &instrtab[var.i];