Improve strength reduction of unsigned comparisons x ==u 0, x !=u 0, etc (#59)

When x is known to be either 0 or 1, comparisons such as
x == 0   x != 0    x == 1    x != 1
can be optimized away.  This optimization was already performed
for signed comparisons.  This commit extends the optimization to
unsigned comparisons as well.

Additionally, for PowerPC only, some unsigned (dis)equality comparisons are
turned into signed comparisons when we know it makes no difference,
i.e. when both arguments are guaranteed not to be pointers.  The
reason is that Asmgen can produce shorter instruction sequences for
some signed equality comparisons than for the corresponding unsigned
comparisons.

It's important to optimize unsigned integer comparisons because casts
to the C99 type _Bool are compiled as x !=u 0 unsigned comparisons.
In particular, cascades of casts to _Bool are now reduced to a single
cast much more often than before.

1 files changed, 31 insertions, 9 deletions

diff --git a/arm/ConstpropOpproof.v b/arm/ConstpropOpproof.v
index 1eec5923..079ba2be 100644
--- a/arm/ConstpropOpproof.v
+++ b/arm/ConstpropOpproof.v
@@ -191,24 +191,46 @@ Proof.
              rs#r = Vundef \/ rs#r = Vint Int.zero \/ rs#r = Vint Int.one).
   { intros. apply vmatch_Uns_1 with bc Ptop. eapply vmatch_ge. eapply vincl_ge; eauto. apply MATCH. }
   unfold make_cmp. case (make_cmp_match c args vl); intros.
-- destruct (Int.eq_dec n Int.one && vincl v1 (Uns Ptop 1)) eqn:E1.
-  simpl in H; inv H. InvBooleans. subst n.
+- unfold make_cmp_imm_eq.
+  destruct (Int.eq_dec n Int.one && vincl v1 (Uns Ptop 1)) eqn:E1.
++ simpl in H; inv H. InvBooleans. subst n.
   exists (rs#r1); split; auto. simpl.
   exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
-  destruct (Int.eq_dec n Int.zero && vincl v1 (Uns Ptop 1)) eqn:E0.
-  simpl in H; inv H. InvBooleans. subst n.
++ destruct (Int.eq_dec n Int.zero && vincl v1 (Uns Ptop 1)) eqn:E0.
+* simpl in H; inv H. InvBooleans. subst n.
   exists (Val.xor rs#r1 (Vint Int.one)); split; auto. simpl.
   exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
-  apply make_cmp_base_correct; auto.
-- destruct (Int.eq_dec n Int.zero && vincl v1 (Uns Ptop 1)) eqn:E0.
-  simpl in H; inv H. InvBooleans. subst n.
+* apply make_cmp_base_correct; auto.
+- unfold make_cmp_imm_ne.
+  destruct (Int.eq_dec n Int.zero && vincl v1 (Uns Ptop 1)) eqn:E0.
++ simpl in H; inv H. InvBooleans. subst n.
   exists (rs#r1); split; auto. simpl.
   exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
++ destruct (Int.eq_dec n Int.one && vincl v1 (Uns Ptop 1)) eqn:E1.
+* simpl in H; inv H. InvBooleans. subst n.
+  exists (Val.xor rs#r1 (Vint Int.one)); split; auto. simpl.
+  exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
+* apply make_cmp_base_correct; auto.
+- unfold make_cmp_imm_eq.
   destruct (Int.eq_dec n Int.one && vincl v1 (Uns Ptop 1)) eqn:E1.
-  simpl in H; inv H. InvBooleans. subst n.
++ simpl in H; inv H. InvBooleans. subst n.
+  exists (rs#r1); split; auto. simpl.
+  exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
++ destruct (Int.eq_dec n Int.zero && vincl v1 (Uns Ptop 1)) eqn:E0.
+* simpl in H; inv H. InvBooleans. subst n.
+  exists (Val.xor rs#r1 (Vint Int.one)); split; auto. simpl.
+  exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
+* apply make_cmp_base_correct; auto.
+- unfold make_cmp_imm_ne.
+  destruct (Int.eq_dec n Int.zero && vincl v1 (Uns Ptop 1)) eqn:E0.
++ simpl in H; inv H. InvBooleans. subst n.
+  exists (rs#r1); split; auto. simpl.
+  exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
++ destruct (Int.eq_dec n Int.one && vincl v1 (Uns Ptop 1)) eqn:E1.
+* simpl in H; inv H. InvBooleans. subst n.
   exists (Val.xor rs#r1 (Vint Int.one)); split; auto. simpl.
   exploit Y; eauto. intros [A | [A | A]]; rewrite A; simpl; auto.
-  apply make_cmp_base_correct; auto.
+* apply make_cmp_base_correct; auto.
 - apply make_cmp_base_correct; auto.
 Qed.