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new 615b07170e avcodec/x86/h264_qpel_8bit: saturate the SSSE3 hv2
intermediate
615b07170e is described below
commit 615b07170e30d79ad5906418ea49f81c3cbd8457
Author: Kieran Kunhya <[email protected]>
AuthorDate: Sun Oct 11 14:51:24 2026 +0100
Commit: kierank <[email protected]>
CommitDate: Mon Oct 12 09:20:21 2026 +0000
avcodec/x86/h264_qpel_8bit: saturate the SSSE3 hv2 intermediate
The second pass of the centre (hv) 6-tap filter computes
((a-b)/4-b+c)/4+c from the first pass's 16-bit sums a, b, c, each in
[-5100, 21420] for 8-bit input (plus a bias that cancels here). The
partial sum (a-b)/4-b+c spans [-33150, 33150], so it can wrap in a 16-bit
lane. The MMX qpel4 and SSE2 hv2 functions already add c with paddsw; the
SSSE3 qpel8/qpel16 hv2 used paddw, and they are the versions every CPU
with SSSE3 runs. With alternating 0/255 stripes of period 3 in both
directions, mc21/mc12/mc22/mc23/mc32 return wrong pixels: a decoder
running this asm reconstructs a different picture from the spec and from
--disable-asm.
Saturating is exact. Saturation happens only when c >= 21038, where the
exact result is at least (8192+21038)/64 = 457 and clips to 255, as does
the saturated path; or when c <= -4719, where both results are negative
and clip to 0. The following add stays inside 16 bits. The assembled
objects differ only in the six opcode bytes (three sites, put and avg).
checkasm now also runs every qpel function on that stripe pattern, in
all six column phases and both polarities.
---
libavcodec/x86/h264_qpel_8bit.asm | 6 +++---
tests/checkasm/h264qpel.c | 27 +++++++++++++++++++++++++++
2 files changed, 30 insertions(+), 3 deletions(-)
diff --git a/libavcodec/x86/h264_qpel_8bit.asm
b/libavcodec/x86/h264_qpel_8bit.asm
index 3aa1f233a0..2bfe6042a1 100644
--- a/libavcodec/x86/h264_qpel_8bit.asm
+++ b/libavcodec/x86/h264_qpel_8bit.asm
@@ -682,7 +682,7 @@ cglobal %1_h264_qpel8_hv2_lowpass, 3,4,6 ; dst, tmp,
dstStride
psubw m0, m1
psraw m0, 2
psubw m0, m1
- paddw m0, m2
+ paddsw m0, m2
psraw m0, 2
paddw m0, m2
psraw m0, 6
@@ -730,8 +730,8 @@ cglobal %1_h264_qpel16_hv2_lowpass, 3,4,8 ; dst, tmp,
dstStride
psraw m3, 2
psubw m0, m1
psubw m3, m4
- paddw m0, m2
- paddw m3, m5
+ paddsw m0, m2
+ paddsw m3, m5
psraw m0, 2
psraw m3, 2
paddw m0, m2
diff --git a/tests/checkasm/h264qpel.c b/tests/checkasm/h264qpel.c
index 6f4a021faf..96e781f78e 100644
--- a/tests/checkasm/h264qpel.c
+++ b/tests/checkasm/h264qpel.c
@@ -45,6 +45,24 @@ static const uint32_t pixel_mask[3] = { 0xffffffff,
0x01ff01ff, 0x03ff03ff };
} \
} while (0)
+/* 0/max stripes with period 3 in both directions take the centre filter's
+ * intermediate sums to their extremes; phase covers every column offset
+ * and both polarities */
+static void fill_stripes(uint8_t *buf0, uint8_t *buf1, int width, int
bit_depth, int phase)
+{
+ int max = (1 << bit_depth) - 1;
+ for (int k = 0; k < BUF_SIZE / SIZEOF_PIXEL; k++) {
+ int x = k % width, y = k / width;
+ int v = ((y % 3 == 1) ^ ("010010"[(x + phase) % 6] - '0') ^ (phase >=
6)) ? max : 0;
+ if (bit_depth == 8) {
+ buf0[k] = buf1[k] = v;
+ } else {
+ AV_WN16A(buf0 + 2 * k, v);
+ AV_WN16A(buf1 + 2 * k, v);
+ }
+ }
+}
+
#define src0 (buf0 + 3 * 2 * 16) /* h264qpel functions read data from negative
src pointer offsets */
#define src1 (buf1 + 3 * 2 * 16)
@@ -73,6 +91,15 @@ void checkasm_check_h264qpel(void)
call_new(dst1, src1, size * SIZEOF_PIXEL);
if (memcmp(buf0, buf1, BUF_SIZE) || memcmp(dst0, dst1,
BUF_SIZE))
fail();
+ for (int phase = 0; phase < 12; phase++) {
+ fill_stripes(buf0, buf1, size, bit_depth, phase);
+ call_ref(dst0, src0, size * SIZEOF_PIXEL);
+ call_new(dst1, src1, size * SIZEOF_PIXEL);
+ if (memcmp(buf0, buf1, BUF_SIZE) || memcmp(dst0,
dst1, BUF_SIZE)) {
+ fail();
+ break;
+ }
+ }
bench_new(dst1, src1, size * SIZEOF_PIXEL);
}
}
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