[PATCH 4/7] tcg/tcg-op: Document hswap() byte pattern

[Philippe Mathieu-Daudé]

Document hswap_i32() and hswap_i64(), added in commit
46be8425ff ("tcg: Implement tcg_gen_{h,w}swap_{i32,i64}").

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
---
 docs/devel/tcg-ops.rst |  4 ++++
 tcg/tcg-op.c           | 26 +++++++++++++++++++-------
 2 files changed, 23 insertions(+), 7 deletions(-)

diff --git a/docs/devel/tcg-ops.rst b/docs/devel/tcg-ops.rst
index 6a166c5665..d9364effd2 100644
--- a/docs/devel/tcg-ops.rst
+++ b/docs/devel/tcg-ops.rst
@@ -486,6 +486,10 @@ Misc
          into 32-bit output *t0*.  Depending on the host, this may be a simple 
shift,
          or may require additional canonicalization.
 
+   * - hswap_i32/i64 *t0*, *t1*
+
+     - | Swap 16-bit halfwords within a 32/64-bit value.
+
 
 Conditional moves
 -----------------
diff --git a/tcg/tcg-op.c b/tcg/tcg-op.c
index f4fe13e040..bb64326a1a 100644
--- a/tcg/tcg-op.c
+++ b/tcg/tcg-op.c
@@ -1091,6 +1091,11 @@ void tcg_gen_bswap32_i32(TCGv_i32 ret, TCGv_i32 arg)
     }
 }
 
+/*
+ * hswap_i32: Swap 16-bit halfwords within a 32-bit value.
+ *
+ * Byte pattern:  hswap_i32(abcd) -> cdab
+ */
 void tcg_gen_hswap_i32(TCGv_i32 ret, TCGv_i32 arg)
 {
     /* Swapping 2 16-bit elements is a rotate. */
@@ -1878,19 +1883,26 @@ void tcg_gen_bswap64_i64(TCGv_i64 ret, TCGv_i64 arg)
     }
 }
 
+/*
+ * hswap_i64: Swap 16-bit halfwords within a 64-bit value.
+ *
+ * See hswap64() in include/qemu/bitops.h
+ *
+ * Byte pattern:  hswap_i64(abcdefgh) -> ghefcdab
+ */
 void tcg_gen_hswap_i64(TCGv_i64 ret, TCGv_i64 arg)
 {
     uint64_t m = 0x0000ffff0000ffffull;
     TCGv_i64 t0 = tcg_temp_ebb_new_i64();
     TCGv_i64 t1 = tcg_temp_ebb_new_i64();
 
-    /* See include/qemu/bitops.h, hswap64. */
-    tcg_gen_rotli_i64(t1, arg, 32);
-    tcg_gen_andi_i64(t0, t1, m);
-    tcg_gen_shli_i64(t0, t0, 16);
-    tcg_gen_shri_i64(t1, t1, 16);
-    tcg_gen_andi_i64(t1, t1, m);
-    tcg_gen_or_i64(ret, t0, t1);
+                                        /* arg = abcdefgh */
+    tcg_gen_rotli_i64(t1, arg, 32);     /*  t1 = efghabcd */
+    tcg_gen_andi_i64(t0, t1, m);        /*  t0 = ..gh..cd */
+    tcg_gen_shli_i64(t0, t0, 16);       /*  t0 = gh..cd.. */
+    tcg_gen_shri_i64(t1, t1, 16);       /*  t1 = ..efghab */
+    tcg_gen_andi_i64(t1, t1, m);        /*  t1 = ..ef..ab */
+    tcg_gen_or_i64(ret, t0, t1);        /* ret = ghefcdab */
 
     tcg_temp_free_i64(t0);
     tcg_temp_free_i64(t1);
-- 
2.41.0