[PULL 40/48] tcg/tcg-op: Document bswap16_i64() byte pattern

[Richard Henderson <richard.henderson@linaro.org>]

From: Philippe Mathieu-Daudé <philmd@linaro.org>

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20230823145542.79633-3-philmd@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
---
 tcg/tcg-op.c | 27 +++++++++++++++++++--------
 1 file changed, 19 insertions(+), 8 deletions(-)

diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
index fc3a0ab7fc..88c7c60ffe 100644
--- a/tcg/tcg-op.c
+++ b/tcg/tcg-op.c
@@ -1767,6 +1767,14 @@ void tcg_gen_ext32u_i64(TCGv_i64 ret, TCGv_i64 arg)
     }
 }
 
+/*
+ * bswap16_i64: 16-bit byte swap on the low bits of a 64-bit value.
+ *
+ * Byte pattern: xxxxxxxxab -> yyyyyyyyba
+ *
+ * With TCG_BSWAP_IZ, x == zero, else undefined.
+ * With TCG_BSWAP_OZ, y == zero, with TCG_BSWAP_OS y == sign, else undefined.
+ */
 void tcg_gen_bswap16_i64(TCGv_i64 ret, TCGv_i64 arg, int flags)
 {
     /* Only one extension flag may be present. */
@@ -1785,22 +1793,25 @@ void tcg_gen_bswap16_i64(TCGv_i64 ret, TCGv_i64 arg, int flags)
         TCGv_i64 t0 = tcg_temp_ebb_new_i64();
         TCGv_i64 t1 = tcg_temp_ebb_new_i64();
 
-        tcg_gen_shri_i64(t0, arg, 8);
+                                            /* arg = ......ab or xxxxxxab */
+        tcg_gen_shri_i64(t0, arg, 8);       /*  t0 = .......a or .xxxxxxa */
         if (!(flags & TCG_BSWAP_IZ)) {
-            tcg_gen_ext8u_i64(t0, t0);
+            tcg_gen_ext8u_i64(t0, t0);      /*  t0 = .......a */
         }
 
         if (flags & TCG_BSWAP_OS) {
-            tcg_gen_shli_i64(t1, arg, 56);
-            tcg_gen_sari_i64(t1, t1, 48);
+            tcg_gen_shli_i64(t1, arg, 56);  /*  t1 = b....... */
+            tcg_gen_sari_i64(t1, t1, 48);   /*  t1 = ssssssb. */
         } else if (flags & TCG_BSWAP_OZ) {
-            tcg_gen_ext8u_i64(t1, arg);
-            tcg_gen_shli_i64(t1, t1, 8);
+            tcg_gen_ext8u_i64(t1, arg);     /*  t1 = .......b */
+            tcg_gen_shli_i64(t1, t1, 8);    /*  t1 = ......b. */
         } else {
-            tcg_gen_shli_i64(t1, arg, 8);
+            tcg_gen_shli_i64(t1, arg, 8);   /*  t1 = xxxxxab. */
         }
 
-        tcg_gen_or_i64(ret, t0, t1);
+        tcg_gen_or_i64(ret, t0, t1);        /* ret = ......ba (OZ) */
+                                            /*       ssssssba (OS) */
+                                            /*       xxxxxaba (no flag) */
         tcg_temp_free_i64(t0);
         tcg_temp_free_i64(t1);
     }
-- 
2.34.1