mem2reg: fix rare edge case with weird control flow

mem2reg was assuming alloca's appeared before all their uses. When this
didn't happen it broke the logic re. sealed blocks. Normally this
doesn't happen in most natural code. Even with gotos, RPO would assure
this in most cases, the exception I found is a goto that jumps into a
loop past a variable declaration that is earlier in the loop. Then even
RPO would keep the alloca itself past the first uses, breaking things.

2 files changed, 30 insertions, 0 deletions

diff --git a/src/ir_mem2reg.c b/src/ir_mem2reg.c
index ae41833..043ac9b 100644
--- a/src/ir_mem2reg.c
+++ b/src/ir_mem2reg.c
@@ -257,6 +257,12 @@ mem2reg(Function *fn)
          do {
             if (use->u == USERJUMP) goto Skip;
             Instr *m = &instrtab[use->u];
+            if (use->blk->id < sb.lastvisit) {
+               /* alloca appears after some of its uses; happens rarely, only
+                * generated with code with gotos into loops. breaks this algo
+                * due to reprocessing sealed blocks */
+               goto Skip;
+            }
             if (oisload(m->op) && (!sz || sz == loadsz(m->op))) {
                sz = loadsz(m->op);
                k = loadcls(m->op);
diff --git a/test/18-goto2.c b/test/18-goto2.c
new file mode 100644
index 0000000..8742d35
--- /dev/null
+++ b/test/18-goto2.c
@@ -0,0 +1,24 @@
+/* EXPECT:
+ok
+*/
+
+void h(int x){}
+
+/* isolated mem2reg bug from something in regalloc: the goto into the loop past
+ * the `int nqueue` declaration caused a miscompilation */
+void test(void) {
+   int curi = 5;
+   goto start2;
+   for (curi; curi >= 0; curi--) {
+      int nqueue;
+   start2:
+      for (nqueue = 2; nqueue > 0; ) {
+         h(nqueue--);
+      }
+   }
+}
+int puts(const char *);
+int main(void) {
+   test();
+   puts("ok");
+}