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docs/environment_variables.md

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+# Introduction
+
+For test purposes, environment variables can be set to control libyuv behavior.  These should only be used for testing, to narrow down bugs or to test performance.
+
+# CPU
+
+By default the cpu is detected and the most advanced form of SIMD is used.  But you can disable instruction sets selectively, or completely, falling back on C code.  Set the variable to 1 to disable the specified instruction set.
+
+    LIBYUV_DISABLE_ASM
+    LIBYUV_DISABLE_X86
+    LIBYUV_DISABLE_SSE2
+    LIBYUV_DISABLE_SSSE3
+    LIBYUV_DISABLE_SSE41
+    LIBYUV_DISABLE_SSE42
+    LIBYUV_DISABLE_AVX
+    LIBYUV_DISABLE_AVX2
+    LIBYUV_DISABLE_AVX3
+    LIBYUV_DISABLE_ERMS
+    LIBYUV_DISABLE_FMA3
+    LIBYUV_DISABLE_MIPS_DSPR2
+    LIBYUV_DISABLE_NEON
+
+# Test Width/Height/Repeat
+
+The unittests default to a small image (32x18) to run fast.  This can be set by environment variable to test a specific resolutions.
+You can also repeat the test a specified number of iterations, allowing benchmarking and profiling.
+
+    set LIBYUV_WIDTH=1280
+    set LIBYUV_HEIGHT=720
+    set LIBYUV_REPEAT=999
+    set LIBYUV_FLAGS=-1
+    set LIBYUV_CPU_INFO=-1