Fix some static analysis minor defects

- fix comparison of uint64_t with SIZE_MAX for platforms where
  uint64_t and size_t are different
- do not compare two floats for equality

src/gainmap.c

@@ -45,12 +45,13 @@ static float avifGetGainMapWeight(float hdrHeadroom, const avifGainMap * gainMap
 {
     const float baseHdrHeadroom = avifUnsignedFractionToFloat(gainMap->baseHdrHeadroom);
     const float alternateHdrHeadroom = avifUnsignedFractionToFloat(gainMap->alternateHdrHeadroom);
-    if (baseHdrHeadroom == alternateHdrHeadroom) {
+    const float diffHdrHeadroom = alternateHdrHeadroom - baseHdrHeadroom;
+    if (diffHdrHeadroom == 0.f) {
         // Do not apply the gain map if the HDR headroom is the same.
         // This case is not handled in the specification and does not make practical sense.
         return 0.0f;
     }
-    const float w = AVIF_CLAMP((hdrHeadroom - baseHdrHeadroom) / (alternateHdrHeadroom - baseHdrHeadroom), 0.0f, 1.0f);
+    const float w = AVIF_CLAMP((hdrHeadroom - baseHdrHeadroom) / diffHdrHeadroom, 0.0f, 1.0f);
     return (alternateHdrHeadroom < baseHdrHeadroom) ? -w : w;
 }
 

src/read.c

@@ -1138,9 +1138,11 @@ static avifResult avifDecoderItemMaxExtent(const avifDecoderItem * item, const a
 
     outExtent->offset = minOffset;
     const uint64_t extentLength = maxOffset - minOffset;
+#if UINT64_MAX != SIZE_MAX
     if (extentLength > SIZE_MAX) {
         return AVIF_RESULT_BMFF_PARSE_FAILED;
     }
+#endif
     outExtent->size = (size_t)extentLength;
     return AVIF_RESULT_OK;
 }
@@ -1956,10 +1958,12 @@ static avifResult avifParseItemLocationBox(avifMeta * meta, const uint8_t * raw,
             }
             uint64_t offset = baseOffset + extentOffset;
             extent->offset = offset;
+#if UINT64_MAX != SIZE_MAX
             if (extentLength > SIZE_MAX) {
                 avifDiagnosticsPrintf(diag, "Item ID [%u] contains an extent length which overflows: [%" PRIu64 "]", itemID, extentLength);
                 return AVIF_RESULT_BMFF_PARSE_FAILED;
             }
+#endif
             extent->size = (size_t)extentLength;
             if (extent->size > SIZE_MAX - item->size) {
                 avifDiagnosticsPrintf(diag,
@@ -4752,9 +4756,11 @@ static avifResult avifExtentMerge(avifExtent * dst, const avifExtent * src)
     const uint64_t maxExtent2 = src->offset + src->size;
     dst->offset = AVIF_MIN(minExtent1, minExtent2);
     const uint64_t extentLength = AVIF_MAX(maxExtent1, maxExtent2) - dst->offset;
+#if UINT64_MAX != SIZE_MAX
     if (extentLength > SIZE_MAX) {
         return AVIF_RESULT_BMFF_PARSE_FAILED;
     }
+#endif
     dst->size = (size_t)extentLength;
     return AVIF_RESULT_OK;
 }
@@ -4824,9 +4830,11 @@ static avifResult avifDecoderPrepareSample(avifDecoder * decoder, avifDecodeSamp
             avifDecoderItem * item;
             AVIF_CHECKRES(avifMetaFindOrCreateItem(decoder->data->meta, sample->itemID, &item));
             avifROData itemContents;
+#if UINT64_MAX != SIZE_MAX
             if (sample->offset > SIZE_MAX) {
                 return AVIF_RESULT_BMFF_PARSE_FAILED;
             }
+#endif
             size_t offset = (size_t)sample->offset;
             avifResult readResult = avifDecoderItemRead(item, decoder->io, &itemContents, offset, bytesToRead, &decoder->diag);
             if (readResult != AVIF_RESULT_OK) {