lib/cpu_features: support disabling CPU features for testing

Make test-only builds of libdeflate support an environmental variable
LIBDEFLATE_DISABLE_CPU_FEATURES that contains a list of CPU features to
disable like "avx512bw,avx2,sse2".

This makes it possible to test all the variants of dynamically
dispatched code without editing the source code.

Note, this environmental variable is not a stable interface, so put the
support for it behind a scary-looking option TEST_SUPPORT__DO_NOT_USE.

Makefile

@@ -60,6 +60,12 @@ override CPPFLAGS += -DFREESTANDING
 LIB_CFLAGS += -ffreestanding -nostdlib
 endif
 
+# Don't use this option except for testing; it isn't a stable interface.
+TEST_SUPPORT__DO_NOT_USE :=
+ifdef TEST_SUPPORT__DO_NOT_USE
+override CPPFLAGS += -DTEST_SUPPORT__DO_NOT_USE
+endif
+
 ##############################################################################
 
 PREFIX ?= /usr/local

lib/arm/cpu_features.c

@@ -35,6 +35,7 @@
  * added to API level 18 for ARM and level 21 for AArch64.
  */
 
+#include "../cpu_features_common.h" /* must be included first */
 #include "cpu_features.h"
 
 #if ARM_CPU_FEATURES_ENABLED
@@ -91,6 +92,11 @@ out:
 	close(fd);
 }
 
+static const struct cpu_feature arm_cpu_feature_table[] = {
+	{ARM_CPU_FEATURE_NEON,		"neon"},
+	{ARM_CPU_FEATURE_PMULL,		"pmull"},
+};
+
 void setup_cpu_features(void)
 {
 	u32 features = 0;
@@ -113,6 +119,9 @@ void setup_cpu_features(void)
 		features |= ARM_CPU_FEATURE_PMULL;
 #endif
 
+	disable_cpu_features_for_testing(&features, arm_cpu_feature_table,
+					 ARRAY_LEN(arm_cpu_feature_table));
+
 	_cpu_features = features | ARM_CPU_FEATURES_KNOWN;
 }
 

lib/cpu_features_common.h

@@ -0,0 +1,83 @@
+/*
+ * cpu_features_common.h - code shared by all lib/$arch/cpu_features.c
+ *
+ * Copyright 2020 Eric Biggers
+ *
+ * Permission is hereby granted, free of charge, to any person
+ * obtaining a copy of this software and associated documentation
+ * files (the "Software"), to deal in the Software without
+ * restriction, including without limitation the rights to use,
+ * copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the
+ * Software is furnished to do so, subject to the following
+ * conditions:
+ *
+ * The above copyright notice and this permission notice shall be
+ * included in all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
+ * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES
+ * OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
+ * NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT
+ * HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY,
+ * WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+ * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR
+ * OTHER DEALINGS IN THE SOFTWARE.
+ */
+
+#if defined(TEST_SUPPORT__DO_NOT_USE) && !defined(FREESTANDING)
+#  define _GNU_SOURCE 1 /* for strdup() and strtok_r() */
+#  include <stdio.h>
+#  include <stdlib.h>
+#  include <string.h>
+#endif
+
+#include "lib_common.h"
+
+struct cpu_feature {
+	u32 bit;
+	const char *name;
+};
+
+#if defined(TEST_SUPPORT__DO_NOT_USE) && !defined(FREESTANDING)
+/* Disable any features that are listed in $LIBDEFLATE_DISABLE_CPU_FEATURES. */
+static inline void
+disable_cpu_features_for_testing(u32 *features,
+				 const struct cpu_feature *feature_table,
+				 size_t feature_table_length)
+{
+	char *env_value, *strbuf, *p, *saveptr = NULL;
+	size_t i;
+
+	env_value = getenv("LIBDEFLATE_DISABLE_CPU_FEATURES");
+	if (!env_value)
+		return;
+	strbuf = strdup(env_value);
+	if (!strbuf)
+		abort();
+	p = strtok_r(strbuf, ",", &saveptr);
+	while (p) {
+		for (i = 0; i < feature_table_length; i++) {
+			if (strcmp(p, feature_table[i].name) == 0) {
+				*features &= ~feature_table[i].bit;
+				break;
+			}
+		}
+		if (i == feature_table_length) {
+			fprintf(stderr,
+				"unrecognized feature in LIBDEFLATE_DISABLE_CPU_FEATURES: \"%s\"\n",
+				p);
+			abort();
+		}
+		p = strtok_r(NULL, ",", &saveptr);
+	}
+	free(strbuf);
+}
+#else /* TEST_SUPPORT__DO_NOT_USE */
+static inline void
+disable_cpu_features_for_testing(u32 *features,
+				 const struct cpu_feature *feature_table,
+				 size_t feature_table_length)
+{
+}
+#endif /* !TEST_SUPPORT__DO_NOT_USE */

lib/x86/cpu_features.c

@@ -25,6 +25,7 @@
  * OTHER DEALINGS IN THE SOFTWARE.
  */
 
+#include "../cpu_features_common.h" /* must be included first */
 #include "cpu_features.h"
 
 #if X86_CPU_FEATURES_ENABLED
@@ -75,6 +76,15 @@ read_xcr(u32 index)
 #define IS_SET(reg, nr)		((reg) & BIT(nr))
 #define IS_ALL_SET(reg, mask)	(((reg) & (mask)) == (mask))
 
+static const struct cpu_feature x86_cpu_feature_table[] = {
+	{X86_CPU_FEATURE_SSE2,		"sse2"},
+	{X86_CPU_FEATURE_PCLMUL,	"pclmul"},
+	{X86_CPU_FEATURE_AVX,		"avx"},
+	{X86_CPU_FEATURE_AVX2,		"avx2"},
+	{X86_CPU_FEATURE_BMI2,		"bmi2"},
+	{X86_CPU_FEATURE_AVX512BW,	"avx512bw"},
+};
+
 /* Initialize _cpu_features with bits for interesting processor features. */
 void setup_cpu_features(void)
 {
@@ -133,6 +143,9 @@ void setup_cpu_features(void)
 		features |= X86_CPU_FEATURE_AVX512BW;
 
 out:
+	disable_cpu_features_for_testing(&features, x86_cpu_feature_table,
+					 ARRAY_LEN(x86_cpu_feature_table));
+
 	_cpu_features = features | X86_CPU_FEATURES_KNOWN;
 }