[FFmpeg-devel] [PATCH] swscale: Break loop-carried dependency enabling parallel out of order execution of the gathers.

[Alan Kelly via ffmpeg-devel <ffmpeg-devel@ffmpeg.org>]

The gather is unmasked but the instruction does a merge into ymm4, which
depends on the value of ymm4 from the previous loop iteration. The
out-of-order scheduler does not know statically that the instruction is
fully unmasked, preventing parallel out-of-order execution of the
gathers.
---
 libswscale/x86/scale_avx2.asm | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/libswscale/x86/scale_avx2.asm b/libswscale/x86/scale_avx2.asm
index b4b852d60b..90ee8b0a0e 100644
--- a/libswscale/x86/scale_avx2.asm
+++ b/libswscale/x86/scale_avx2.asm
@@ -68,8 +68,10 @@ cglobal hscale8to15_%1, 7, 9, 16, pos0, dst, w, srcmem, filter, fltpos, fltsize,
 .innerloop:
 %endif
     vpcmpeqd  m13, m13
+    pxor m3, m3  ; break loop-carried dependency
     vpgatherdd m3,[srcmemq + m1], m13
     vpcmpeqd  m13, m13
+    pxor m4, m4  ; break loop-carried dependency
     vpgatherdd m4,[srcmemq + m2], m13
     vpunpcklbw m5, m3, m0
     vpunpckhbw m6, m3, m0
@@ -119,6 +121,7 @@ cglobal hscale8to15_%1, 7, 9, 16, pos0, dst, w, srcmem, filter, fltpos, fltsize,
 .tail_innerloop:
 %endif
     vpcmpeqd  xm13, xm13
+    pxor m3, m3  ; break loop-carried dependency
     vpgatherdd xm3,[srcmemq + xm1], xm13
     vpunpcklbw xm5, xm3, xm0
     vpunpckhbw xm6, xm3, xm0
-- 
2.50.1.565.gc32cd1483b-goog

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