Expand vec_perm_indices::series_p comment

2018-01-30  Richard Sandiford  <richard.sandiford@linaro.org>

gcc/
	* vec-perm-indices.c (vec_perm_indices::series_p): Give examples
	of usage.

From-SVN: r257176

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index aafaa36975ab3ba06a046c7426ffe5ee58a5fac7..4aecf47e45a5127326085683a43e3d4792ec55f3 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,8 @@
+2018-01-30  Richard Sandiford  <richard.sandiford@linaro.org>
+
+       * vec-perm-indices.c (vec_perm_indices::series_p): Give examples
+       of usage.
+
 2018-01-29  Michael Meissner  <meissner@linux.vnet.ibm.com>
 
        PR target/81550

diff --git a/gcc/vec-perm-indices.c b/gcc/vec-perm-indices.c
index 53dbe0daedd8a014bea45c80a247f4e507f33902..92489613c1b72e6015511c4cbd61c86f0012c4f6 100644 (file)
--- a/gcc/vec-perm-indices.c
+++ b/gcc/vec-perm-indices.c
@@ -114,7 +114,18 @@ vec_perm_indices::rotate_inputs (int delta)
 }
 
 /* Return true if index OUT_BASE + I * OUT_STEP selects input
-   element IN_BASE + I * IN_STEP.  */
+   element IN_BASE + I * IN_STEP.  For example, the call to test
+   whether a permute reverses a vector of N elements would be:
+
+     series_p (0, 1, N - 1, -1)
+
+   which would return true for { N - 1, N - 2, N - 3, ... }.
+   The calls to test for an interleaving of elements starting
+   at N1 and N2 would be:
+
+     series_p (0, 2, N1, 1) && series_p (1, 2, N2, 1).
+
+   which would return true for { N1, N2, N1 + 1, N2 + 1, ... }.  */
 
 bool
 vec_perm_indices::series_p (unsigned int out_base, unsigned int out_step,