Fix handling of mirroring upscaling in simple bilinear upscaler

Calculates the correct offsets and coordinate transforms for the
intermediate buffer. This means we can conceptually simplify our
path switches instead of having downscale routines handling mirrored
upscaling.

Change-Id: I60efa7feaba80165672ca0ce064515fdf620869d
Reviewed-by: Eirik Aavitsland <eirik.aavitsland@qt.io>

1 files changed, 10 insertions, 7 deletions

diff --git a/src/gui/painting/qdrawhelper_avx2.cpp b/src/gui/painting/qdrawhelper_avx2.cpp
index a378fca749..d6c3319c76 100644
--- a/src/gui/painting/qdrawhelper_avx2.cpp
+++ b/src/gui/painting/qdrawhelper_avx2.cpp
@@ -420,10 +420,14 @@ void QT_FASTCALL fetchTransformedBilinearARGB32PM_simple_upscale_helper_avx2(uin
     const uint *s1 = (const uint *)image.scanLine(y1);
     const uint *s2 = (const uint *)image.scanLine(y2);
 
-    int disty = (fy & 0x0000ffff) >> 8;
-    int idisty = 256 - disty;
-    int x = fx >> 16;
-    int length = end - b;
+    const int disty = (fy & 0x0000ffff) >> 8;
+    const int idisty = 256 - disty;
+    const int length = end - b;
+
+    // The intermediate buffer is generated in the positive direction
+    const int adjust = (fdx < 0) ? fdx * length : 0;
+    const int offset = (fx + adjust) >> 16;
+    int x = offset;
 
     // The idea is first to do the interpolation between the row s1 and the row s2
     // into an intermediate buffer, then we interpolate between two pixel of this buffer.
@@ -433,7 +437,7 @@ void QT_FASTCALL fetchTransformedBilinearARGB32PM_simple_upscale_helper_avx2(uin
     // +1 for the last pixel to interpolate with, and +1 for rounding errors.
     quint32 intermediate_buffer[2][BufferSize + 2];
     // count is the size used in the intermediate_buffer.
-    int count = (qint64(length) * fdx + FixedScale - 1) / FixedScale + 2;
+    int count = (qint64(length) * qAbs(fdx) + FixedScale - 1) / FixedScale + 2;
     Q_ASSERT(count <= BufferSize + 2); //length is supposed to be <= buffer_size and data->m11 < 1 in this case
     int f = 0;
     int lim = qMin(count, image.x2 - x);
@@ -492,8 +496,7 @@ void QT_FASTCALL fetchTransformedBilinearARGB32PM_simple_upscale_helper_avx2(uin
         x++;
     }
     // Now interpolate the values from the intermediate_buffer to get the final result.
-    fx &= FixedScale - 1;
-    Q_ASSERT((fx >> 16) == 0);
+    fx -= offset * FixedScale;
 
     const __m128i v_fdx = _mm_set1_epi32(fdx * 4);
     const __m128i v_blend = _mm_set1_epi32(0x00800080);