sysv: document vaargs stack layout stuff

1 files changed, 29 insertions, 3 deletions

diff --git a/amd64/sysv.c b/amd64/sysv.c
index af0ade0..db88692 100644
--- a/amd64/sysv.c
+++ b/amd64/sysv.c
@@ -140,6 +140,30 @@ abiret(short r[2], uchar cls[2], int *ni, union irtype typ)
    return ret;
 }
 
+/* Layout of va_list:
+ * struct {
+ *    ( 0) unsigned int gp_offset;
+ *    ( 4) unsigned int fp_offset;
+ *    ( 8) void *overflow_arg_area;
+ *    (16) void *reg_save_area;
+ * }
+ * Layout of register save area (align 16):
+ *    reg  off
+ *    rdi   0
+ *    rsi   8
+ *    rdx  16
+ *    rcx  24
+ *    r8   32
+ *    r9   40
+ *    xmm0 48
+ *    xmm1 64
+ *    ...
+ *  in amd64/emit xvaprologue generates the code to save the registers to a stack slot
+ *  there only needs to be one xvaprologue if there's any vastart instrs, and it has to be
+ *  at the beginning of the function (before IR generated by regalloc can touch any registers)
+ *  then vastart can initialize va_list.reg_save_area with a pointer to that
+ */
+
 static void
 vastart(struct function *fn, struct block *blk, int *curi)
 {
@@ -148,9 +172,11 @@ vastart(struct function *fn, struct block *blk, int *curi)
    struct instr *ins = &instrtab[blk->ins.p[*curi]];
    union ref ap = ins->l, src, dst;
    assert(ins->op == Ovastart);
-   /* add xvaprologue if not there yet, which must be the first instruction in the function */
-   if (fn->entry->ins.n > 0 && instrtab[fn->entry->ins.p[0]].op == Oxvaprologue) {
-      rsave = mkref(RTMP, fn->entry->ins.p[0]);
+   /* add xvaprologue if not there yet, which must be the first
+    * real instruction in the function (following alloca) */
+   if (fn->entry->ins.n > 1 && instrtab[fn->entry->ins.p[1]].op == Oxvaprologue) {
+      rsave = mkref(RTMP, fn->entry->ins.p[0]); /* alloca instruction */
+      assert(instrtab[rsave.i].op == Oalloca16);
    } else {
       rsave = insertinstr(fn->entry, 0, mkalloca(192, 16));
       insertinstr(fn->entry, 1, mkinstr(Oxvaprologue, 0, rsave, .keep=1));