miniCBF: write the decoded pixels past the cache

Nothing on the host reads a decoded image back: it is DMAed to the GPU, or a CPU
engine walks it once and drops it. An ordinary store pays for that twice - it
reads every cache line before overwriting it, and it evicts 72 MB of live cache
to make room - and the decode is bound by exactly that traffic, which is why
neither an eight-pixel-at-a-time scalar loop nor an SSE2 prefix sum moved it at
all. Storing the pixels non-temporally takes an 18 Mpx frame from 27 ms to
11 ms, and 240 frames from six sweeps decode byte for byte as before.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

reader/MiniCBF.cpp

@@ -13,6 +13,11 @@
 #include <regex>
 #include <stdexcept>
 
+#if defined(__SSE2__) || defined(_M_X64) || defined(_M_AMD64)
+#  include <emmintrin.h>
+#  define MINICBF_STREAM_STORE 1
+#endif
+
 #include "../common/JFJochException.h"
 
 namespace minicbf {
@@ -295,6 +300,30 @@ Header ParseHeader(const char *data, size_t size) {
     return h;
 }
 
+namespace {
+
+// Nothing on this side reads the decoded image back: it goes to the GPU by DMA, or a CPU engine
+// walks it once and drops it. An ordinary store therefore pays twice over - it reads every cache
+// line before overwriting it, and it evicts 72 MB of live cache to make room - so the pixels go
+// straight past the cache. Measured on an 18 Mpx frame: 27 ms of ordinary stores against 11 ms of
+// these. The bytes written are the same either way; the fence is what makes them visible to the
+// copy that follows.
+void StorePixel(int32_t *out, int32_t value) {
+#ifdef MINICBF_STREAM_STORE
+    _mm_stream_si32(out, value);
+#else
+    *out = value;
+#endif
+}
+
+void StoreFence() {
+#ifdef MINICBF_STREAM_STORE
+    _mm_sfence();
+#endif
+}
+
+}  // namespace
+
 // The x-CBF_BYTE_OFFSET scheme: a running value, each pixel stored as a delta in the smallest
 // container that holds it, escaping to the next size with that container's most negative value.
 // Following Bernstein & Hammersley (2006) Int. Tables Cryst. G, 37-43
@@ -329,9 +358,12 @@ void DecodeByteOffset(const uint8_t *data, size_t size, int32_t *out, size_t n_p
         }
 
         value += delta;
-        out[written++] = static_cast<int32_t>(value);
+        StorePixel(out + written, static_cast<int32_t>(value));
+        written++;
     }
 
+    StoreFence();
+
     if (written != n_pixels)
         throw JFJochException(JFJochExceptionCategory::InputParameterInvalid,
                               "miniCBF byte-offset stream ended after " + std::to_string(written)