[loong64] Fix missing rounding in loong64 scaled_mult implementation

The reference semantics of scaled_mult include rounding, but the
original implementation did not do so. This is triggering an SkASSERT
in the unit test case FilterResult_raster_RescaleWithTransform, from
constrained_add's debug checks.

The fixed implementation bumps the cost of each scaled_mult from 2
to 5 instruction-count-wise (5 to 8 clock-cycle-wise with the LA464
and LA664 micro-architectures), due to unavailability of rounding
multiply operations in current LoongArch spec. However the computation
now matches the reference scalar semantics, and proper testing of
debug builds is now possible.

Change-Id: I45e43a7a7e6d50b4c32c5e69a6d1d7de341eccf1

[Kaiyang Wu: backport to webkit2gtk]

Link: https://skia-review.googlesource.com/c/skia/+/908136
Signed-off-by: Kaiyang Wu <[email protected]>

Author: OriginCode

Source/ThirdParty/skia/src/opts/SkRasterPipeline_opts.h

@@ -5532,11 +5532,19 @@ SI I16 scaled_mult(I16 a, I16 b) {
 #elif defined(SKRP_CPU_NEON)
     return vqrdmulhq_s16(a, b);
 #elif defined(SKRP_CPU_LASX)
-    I16 res = __lasx_xvmuh_h(a, b);
-    return __lasx_xvslli_h(res, 1);
+    Vec<8, int32_t> even = (Vec<8, int32_t>)__lasx_xvmulwev_w_h((__m256i)a, (__m256i)b);
+    Vec<8, int32_t> odd = (Vec<8, int32_t>)__lasx_xvmulwod_w_h((__m256i)a, (__m256i)b);
+    Vec<8, int32_t> roundingTerm = (Vec<8, int32_t>)__lasx_xvldi(-0xec0);  // v8i32(0x40 << 8)
+    even = (even + roundingTerm) >> 15;
+    odd = (odd + roundingTerm) >> 15;
+    return (I16)__lasx_xvpackev_h((__m256i)odd, (__m256i)even);
 #elif defined(SKRP_CPU_LSX)
-    I16 res = __lsx_vmuh_h(a, b);
-    return __lsx_vslli_h(res, 1);
+    Vec<4, int32_t> even = (Vec<4, int32_t>)__lsx_vmulwev_w_h((__m128i)a, (__m128i)b);
+    Vec<4, int32_t> odd = (Vec<4, int32_t>)__lsx_vmulwod_w_h((__m128i)a, (__m128i)b);
+    Vec<4, int32_t> roundingTerm = (Vec<4, int32_t>)__lsx_vldi(-0xec0);  // v4i32(0x40 << 8)
+    even = (even + roundingTerm) >> 15;
+    odd = (odd + roundingTerm) >> 15;
+    return (I16)__lsx_vpackev_h((__m128i)odd, (__m128i)even);
 #else
     const I32 roundingTerm = I32_(1 << 14);
     return cast<I16>((cast<I32>(a) * cast<I32>(b) + roundingTerm) >> 15);