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simd/README.md
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simd/README.md
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libjpeg-turbo SIMD Extensions
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=============================
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This directory contains the libjpeg-turbo SIMD extensions, hand-coded assembly
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or compiler intrinsics modules that plug into the modular interfaces of the
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libjpeg API library and use various SIMD instruction sets to accelerate most of
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the 8-bit-per-sample lossy JPEG compression and decompression algorithms.
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(Note that, since the TurboJPEG API library wraps the libjpeg API library, it
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uses the libjpeg-turbo SIMD extensions implicitly.)
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Algorithm Coverage
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------------------
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The following 8-bit-per-sample lossy JPEG compression algorithms are currently
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implemented as SIMD modules:
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- Color Conversion (see [jccolor.c](../src/jccolor.c))
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* RGB-to-YCbCr Color Conversion
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* RGB-to-Grayscale Color Conversion
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* NULL Color Conversion (infrequently used)
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- Downsampling (see [jcsample.c](../src/jcsample.c))
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* H2V1 (4:2:2) Downsampling
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* H2V2 (4:2:0) Downsampling
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* H2V2 (4:2:0) Smooth Downsampling (infrequently used)
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- Sample Conversion (see [jcdctmgr.c](../src/jcdctmgr.c))
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* Integer Sample Conversion
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* Floating Point Sample Conversion (legacy feature)
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- Forward DCT (see [jcdctmgr.c](../src/jcdctmgr.c))
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* Accurate Integer Forward DCT
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* Fast Integer Forward DCT (legacy feature)
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* Floating Point Forward DCT (legacy feature)
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- Quantization (see [jcdctmgr.c](../src/jcdctmgr.c))
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* Integer Quantization
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* Floating Point Quantization (legacy feature)
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- Entropy Encoding
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* Huffman Encoding (see [jchuff.c](../src/jchuff.c))
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* Progressive Huffman Encoding (see [jcphuff.c](../src/jcphuff.c))
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The following 8-bit-per-sample lossy JPEG decompression algorithms are
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currently implemented as SIMD modules:
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- Inverse DCT (see [jddctmgr.c](../src/jddctmgr.c))
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* Accurate Integer Inverse DCT
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* Fast Integer Inverse DCT (legacy feature)
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* Floating Point Inverse DCT (legacy feature)
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* 2x2 (1/4 Scaling) Integer Inverse DCT (infrequently used)
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* 4x4 (1/2 Scaling) Integer Inverse DCT (infrequently used)
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* 6x6 (3/4 Scaling) Integer Inverse DCT (infrequently used)
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* 12x12 (3/2 Scaling) Integer Inverse DCT (infrequently used)
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- Upsampling (see [jdsample.c](../src/jdsample.c))
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* H2V1 (4:2:2) Fancy (Smooth) Upsampling
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* H2V2 (4:2:0) Fancy (Smooth) Upsampling
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* H1V2 (4:4:0) Fancy (Smooth) Upsampling
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* H2V1 (4:2:2) Plain Upsampling (infrequently used)
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* H2V2 (4:2:0) Plain Upsampling (infrequently used)
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* Integer Upsampling (infrequently used)
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- Merged Upsampling (see [jdmerge.c](../src/jdmerge.c))
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* H2V1 (4:2:2) Merged Upsampling/Color Conversion
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* H2V2 (4:2:0) Merged Upsampling/Color Conversion
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- Color Deconversion (see [jdcolor.c](../src/jdcolor.c))
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* YCbCr-to-RGB Color Conversion
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* YCbCr-to-RGB565 Color Conversion (infrequently used)
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Refer to <https://libjpeg-turbo.org/About/SIMDCoverage> for a list of SIMD
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modules that are implemented for the algorithms above using specific SIMD
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instruction sets.
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Legacy features are features that were designed to work around hardware
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performance limitations that no longer exist. They generally have little or no
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utility on modern hardware and are retained only for backward compatibility
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with libjpeg.
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Infrequently used features may be useful for specific applications but are not
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the "common case" for JPEG compression and decompression.
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SIMD Dispatcher Operation
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-------------------------
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When initializing a particular compression or decompression module (which
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occurs during `jpeg_start_compress()`, `jpeg_start_decompress()`,
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`tj3Compress*()`, or `tj3Decompress*()`), the libjpeg API library calls a SIMD
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dispatcher function (`jsimd_set_*()`) for each algorithm listed above. The
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SIMD dispatcher functions are defined in [jsimd.h](jsimd.h),
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[jsimddct.h](jsimddct.h), and [jsimd.c](jsimd.c). Each function
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- determines which SIMD instruction sets are available for the current
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architecture (if such has not already been determined for the current API
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instance),
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- determines whether a compatible SIMD module exists for the specified
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algorithm, and
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- (if so) plugs the best available SIMD module for that algorithm into the
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appropriate compression or decompression module, thus causing a SIMD
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implementation of the algorithm to be used instead of the scalar/C
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implementation.
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You can use environment variables to override the dispatchers' choice of SIMD
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modules:
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- `JSIMD_FORCENONE=1` disables all SIMD modules.
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- `JSIMD_NOHUFFENC=1` disables only the Huffman encoding SIMD modules.
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- `JSIMD_FORCESSE2=1` (x86) enables only the SSE2 and SSE SIMD modules, even if
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the CPU supports newer instruction sets.
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- `JSIMD_FORCESSE=1` (i386) enables only the SSE and MMX SIMD modules, even if
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the CPU supports newer instruction sets.
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- `JSIMD_FORCEMMX=1` (i386) enables only the MMX SIMD modules, even if the CPU
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supports newer instruction sets.
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- `JSIMD_FORCENEON=1` (AArch32) force-enables the Neon SIMD modules, bypassing
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**/proc/cpuinfo** feature detection (which may be unreliable in QEMU and
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other emulation/virtualization environments.)
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- `JSIMD_FORCEDSPR2=1` (MIPS) force-enables the DSPr2 SIMD modules, bypassing
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**/proc/cpuinfo** feature detection (which may be unreliable in QEMU and
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other emulation/virtualization environments.)
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- `JSIMD_FORCEMMI=1` (Loongson) force-enables the MMI SIMD modules, bypassing
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**/proc/cpuinfo** feature detection (which may be unreliable in QEMU and
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other emulation/virtualization environments.)
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The **simdcoverage** program reports which SIMD modules will be used, taking
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into account the current architecture, detected CPU features, and
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aforementioned overrides.
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SIMD Module Performance Profiling
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---------------------------------
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When built with the `WITH_PROFILE` CMake variable enabled, the libjpeg API
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library measures the average throughput of each lossy JPEG algorithm as an
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image is compressed or decompressed, and it prints the results to the command
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line when `jpeg_destroy_compress()`, `jpeg_destroy_decompress()`, or
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`tj3Destroy()` is called. This allows developers to easily study the
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performance of each SIMD module in isolation and compare it to the performance
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of the corresponding scalar/C module (or a previous implementation of the same
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SIMD module.)
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The most effective way to use the profiling feature is with the TJBench application
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and a suitably large image, such as one of the 8-bit RGB images from
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[imagecompression.info](http://imagecompression.info/test_images). Use one of
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the following command lines to obtain the performance of specific algorithms.
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(Adjust the warmup and benchmark times to suit your needs.)
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422`
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* "Color conversion" reports the performance of the RGB-to-YCbCr Color
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Conversion algorithm.
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* "Downsampling" reports the performance of the H2V1 (4:2:2) Downsampling
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algorithm.
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* "Sample conversion" reports the performance of the Integer Sample
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Conversion algorithm.
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* "Forward DCT" reports the performance of the Accurate Integer Forward DCT
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algorithm.
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* "Quantization" reports the performance of the Integer Quantization
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algorithm.
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* "Entropy encoding" reports the performance of the Huffman Encoding
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algorithm.
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* "Inverse DCT" reports the performance of the Accurate Integer Inverse DCT
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algorithm.
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* "Upsampling" reports the performance of the H2V1 (4:2:2) Fancy (Smooth)
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Upsampling algorithm.
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* "Color deconversion" reports the performance of the YCbCr-to-RGB Color
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Conversion algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp gray`
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* "Color conversion" reports the performance of the RGB-to-Grayscale Color
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Conversion algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 420`
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* "Downsampling" reports the performance of the H2V2 (4:2:0) Downsampling
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algorithm.
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* "Upsampling" reports the performance of the H2V2 (4:2:0) Fancy (Smooth)
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Upsampling algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 440`
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* "Upsampling" reports the performance of the H1V2 (4:4:0) Fancy (Smooth)
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Upsampling algorithm.
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- `tjbench {image}.ppm 95 -cmyk -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -nosmooth`
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* "Upsampling" reports the performance of the H2V1 (4:2:2) Plain Upsampling
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algorithm.
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- `tjbench {image}.ppm 95 -cmyk -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 420 -nosmooth`
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* "Upsampling" reports the performance of the H2V2 (4:2:0) Plain Upsampling
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algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 440 -nosmooth`
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* "Upsampling" reports the performance of the Integer Upsampling algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -nosmooth`
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* "Merged upsampling" reports the performance of the H2V1 (4:2:2) Merged
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Upsampling/Color Conversion algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 420 -nosmooth`
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* "Merged upsampling" reports the performance of the H2V2 (4:2:0) Merged
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Upsampling/Color Conversion algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -progressive`
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* "Entropy encoding" reports the performance of the Progressive Huffman
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Encoding algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -scale 1/4`
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* "Inverse DCT" reports the performance of the 2x2 (1/4 Scaling) Integer
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Inverse DCT algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -scale 1/2`
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* "Inverse DCT" reports the performance of the 4x4 (1/2 Scaling) Integer
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Inverse DCT algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -scale 3/4`
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* "Inverse DCT" reports the performance of the 6x6 (3/4 Scaling) Integer
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Inverse DCT algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -scale 3/2`
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* "Inverse DCT" reports the performance of the 12x12 (3/2 Scaling) Integer
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Inverse DCT algorithm.
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- `tjbench {image}.ppm 95 -rgb -quiet -nowrite -benchtime 10 -warmup 10 -subsamp 422 -dct fast`
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* "Forward DCT" reports the performance of the Fast Integer Forward DCT
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algorithm.
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* "Inverse DCT" reports the performance of the Fast Integer Inverse DCT
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algorithm.
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