Make huffman decoding slightly faster using evil looking macro

lib/nghttp2_hd_huffman.c

@@ -168,6 +168,23 @@ void nghttp2_hd_huff_decode_context_init(nghttp2_hd_huff_decode_context *ctx) {
   ctx->accept = 1;
 }
 
+/* Use macro to make the code simpler..., but error case is tricky.
+   We spent most of the CPU in decoding, so we are doing this
+   thing. */
+#define hd_huff_decode_sym_emit(bufs, sym, avail)                              \
+  do {                                                                         \
+    if ((avail)) {                                                             \
+      nghttp2_bufs_fast_addb((bufs), (sym));                                   \
+      --(avail);                                                               \
+    } else {                                                                   \
+      rv = nghttp2_bufs_addb((bufs), (sym));                                   \
+      if (rv != 0) {                                                           \
+        return rv;                                                             \
+      }                                                                        \
+      (avail) = nghttp2_bufs_cur_avail((bufs));                                \
+    }                                                                          \
+  } while (0)
+
 ssize_t nghttp2_hd_huff_decode(nghttp2_hd_huff_decode_context *ctx,
                                nghttp2_bufs *bufs, const uint8_t *src,
                                size_t srclen, int final) {
@@ -180,30 +197,28 @@ ssize_t nghttp2_hd_huff_decode(nghttp2_hd_huff_decode_context *ctx,
   /* We use the decoding algorithm described in
      http://graphics.ics.uci.edu/pub/Prefix.pdf */
   for (i = 0; i < srclen; ++i) {
-    uint8_t in = src[i] >> 4;
-    for (j = 0; j < 2; ++j) {
-      const nghttp2_huff_decode *t;
+    const nghttp2_huff_decode *t;
 
-      t = &huff_decode_table[ctx->state][in];
-      if (t->flags & NGHTTP2_HUFF_FAIL) {
-        return NGHTTP2_ERR_HEADER_COMP;
-      }
-      if (t->flags & NGHTTP2_HUFF_SYM) {
-        if (avail) {
-          nghttp2_bufs_fast_addb(bufs, t->sym);
-          --avail;
-        } else {
-          rv = nghttp2_bufs_addb(bufs, t->sym);
-          if (rv != 0) {
-            return rv;
-          }
-          avail = nghttp2_bufs_cur_avail(bufs);
-        }
-      }
-      ctx->state = t->state;
-      ctx->accept = (t->flags & NGHTTP2_HUFF_ACCEPTED) != 0;
-      in = src[i] & 0xf;
+    t = &huff_decode_table[ctx->state][src[i] >> 4];
+    if (t->flags & NGHTTP2_HUFF_FAIL) {
+      return NGHTTP2_ERR_HEADER_COMP;
+    }
+    if (t->flags & NGHTTP2_HUFF_SYM) {
+      /* this is macro, and may return from this function on error */
+      hd_huff_decode_sym_emit(bufs, t->sym, avail);
     }
+
+    t = &huff_decode_table[t->state][src[i] & 0xf];
+    if (t->flags & NGHTTP2_HUFF_FAIL) {
+      return NGHTTP2_ERR_HEADER_COMP;
+    }
+    if (t->flags & NGHTTP2_HUFF_SYM) {
+      /* this is macro, and may return from this function on error */
+      hd_huff_decode_sym_emit(bufs, t->sym, avail);
+    }
+
+    ctx->state = t->state;
+    ctx->accept = (t->flags & NGHTTP2_HUFF_ACCEPTED) != 0;
   }
   if (final && !ctx->accept) {
     return NGHTTP2_ERR_HEADER_COMP;