Add the start of a peephole optimizer.

    It only does one optimization right now -- the elimination
    of dumb jumps. Eventually it will do more.

1 files changed, 86 insertions, 0 deletions

diff --git a/6/peep.c b/6/peep.c
new file mode 100644
index 0000000..b7cbcb5
--- /dev/null
+++ b/6/peep.c
@@ -0,0 +1,86 @@
+#include <stdlib.h>
+#include <stdio.h>
+#include <stdint.h>
+#include <stdarg.h>
+#include <ctype.h>
+#include <string.h>
+#include <assert.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+#include "parse.h"
+#include "opt.h"
+#include "asm.h"
+
+
+/* we sometimes leave dead code generated after 
+ * a non-conditional jump. This code scans for
+ * the non-conditional exit from the BB, and truncates
+ * at that point */
+static void elimdead(Isel *s, Asmbb *bb)
+{
+    size_t i;
+
+    for (i = 0; i < bb->ni; i++) {
+        if (bb->il[i]->op == Ijmp) {
+            i++;
+            break;
+        }
+    }
+    bb->ni = i;
+}
+
+/* checks for of dumb jump code.
+ * nop jumps:
+ *      jmp .l1
+ *      .l1:
+ * TODO: check for jumps over jumps.
+ */     
+static void elimnopjmp(Isel *s, Asmbb *bb, size_t idx)
+{
+    Insn *jmp;
+    Loc *targ;
+    Asmbb *nextbb;
+    size_t i;
+
+    /* skip empty bbs */
+    if (!bb->ni)
+        return;
+    /* find the target of the last unconditional
+     * jump in the bb */
+    targ = NULL;
+    if (bb->il[bb->ni - 1]->op == Ijmp) {
+        jmp = bb->il[bb->ni - 1];
+        if (jmp->args[0]->type == Loclbl)
+            targ = jmp->args[0];
+    }
+    if (!targ)
+        return;
+    
+    /* figure out if it's somewhere in the head of the next bb */
+    if (idx < s->nbb)
+        nextbb = s->bb[idx + 1];
+    for (i = 0; i < nextbb->nlbls; i++) {
+        if (!strcmp(nextbb->lbls[i], targ->lbl)) {
+            bb->ni--;
+            break;
+        }
+    }
+}
+
+static void bbpeep(Isel *s, Asmbb *bb, size_t idx)
+{
+    elimdead(s, bb);
+    elimnopjmp(s, bb, idx);
+}
+
+void peep(Isel *s)
+{
+    size_t i;
+
+    for (i = 0; i < s->nbb; i++) {
+        bbpeep(s, s->bb[i], i);
+    }
+}