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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