[FFmpeg-cvslog] aarch64: hevc_idct: Fix overflows in idct_dc

[Martin Storsjö]

ffmpeg | branch: master | Martin Storsjö <martin at martin.st> | Mon May 17 12:48:03 2021 +0300| [f27e3ccf06ee19935d160164ca4a02f28cfc2a27] | committer: Martin Storsjö

aarch64: hevc_idct: Fix overflows in idct_dc

This is marginally slower, but correct for all input values.
The previous implementation failed with certain input seeds, e.g.
"checkasm --test=hevc_idct 98".

Signed-off-by: Martin Storsjö <martin at martin.st>

> http://git.videolan.org/gitweb.cgi/ffmpeg.git/?a=commit;h=f27e3ccf06ee19935d160164ca4a02f28cfc2a27
---

 libavcodec/aarch64/hevcdsp_idct_neon.S | 11 +++++------
 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/libavcodec/aarch64/hevcdsp_idct_neon.S b/libavcodec/aarch64/hevcdsp_idct_neon.S
index 28c11e632c..0869431294 100644
--- a/libavcodec/aarch64/hevcdsp_idct_neon.S
+++ b/libavcodec/aarch64/hevcdsp_idct_neon.S
@@ -573,14 +573,13 @@ idct_16x16 10
 // void ff_hevc_idct_NxN_dc_DEPTH_neon(int16_t *coeffs)
 .macro idct_dc size, bitdepth
 function ff_hevc_idct_\size\()x\size\()_dc_\bitdepth\()_neon, export=1
-        movi          v1.8h,  #((1 << (14 - \bitdepth))+1)
         ld1r         {v4.8h}, [x0]
-        add           v4.8h,  v4.8h,  v1.8h
-        sshr          v0.8h,  v4.8h,  #(15 - \bitdepth)
-        sshr          v1.8h,  v4.8h,  #(15 - \bitdepth)
+        srshr         v4.8h,  v4.8h,  #1
+        srshr         v0.8h,  v4.8h,  #(14 - \bitdepth)
+        srshr         v1.8h,  v4.8h,  #(14 - \bitdepth)
 .if \size > 4
-        sshr          v2.8h,  v4.8h,  #(15 - \bitdepth)
-        sshr          v3.8h,  v4.8h,  #(15 - \bitdepth)
+        srshr         v2.8h,  v4.8h,  #(14 - \bitdepth)
+        srshr         v3.8h,  v4.8h,  #(14 - \bitdepth)
 .if \size > 16 /* dc 32x32 */
         mov              x2,  #4
 1: