Introduce ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY in absl/base/options.h
to provide a centralized policy for hardware-accelerated
implementations in headers.

This option addresses ODR violations that occur when Abseil headers
use compile-time feature detection (such as __SSE4_2__) in inline
functions. If translation units are compiled with inconsistent CPU
architecture flags (e.g., -march=native vs -march=generic) and linked
together, they may disagree on the implementation of these inline
functions. This can lead to crashes or silent data corruption in
hashing.

The new option provides three levels of control:
- 0: Portable. Forces software-only implementations in headers,
  guaranteeing ABI safety across mixed translation units.
- 1: Required. Forces hardware-accelerated implementations in
  headers. The build will fail if the required instructions are not
  enabled in the compiler environment.
- 2: Auto-detect. Selects the best available implementation based on
  compiler flags, but can't guarantee safety if translation units are
  compiled with inconsistent flags.

PiperOrigin-RevId: 896569507
Change-Id: Ifcbc1f3980883feeaa4f05f845ce32310ca7d533

absl/base/options.h

diff --git a/absl/base/options.h b/absl/base/options.h
index cfacc6e..033eac3 100644
--- a/absl/base/options.h
+++ b/absl/base/options.h
@@ -183,4 +183,30 @@
 
 #define ABSL_OPTION_HARDENED 0
 
+// ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY
+//
+// This option controls whether Abseil is allowed to use non-portable
+// hardware-accelerated implementations in headers (where they are typically
+// inlined into the caller's translation unit).
+//
+// Using such optimizations in headers can lead to One Definition Rule (ODR)
+// violations if different translation units are built with different CPU
+// architecture flags (e.g., -march=native vs -march=generic) and linked
+// together.
+//
+// A value of 0 means to use the portable software implementation in headers.
+// This provides the best ODR guarantees when linking code built with
+// inconsistent flags, but may be slower.
+//
+// A value of 1 means that the implementation requires the use of a
+// hardware-accelerated implementation. This requires the compiler environment
+// to support these instructions; otherwise, the build will fail.
+//
+// A value of 2 means to select the best available implementation based on
+// the compiler flags, but can't guarantee ODR safety if translation units are
+// built with inconsistent flags.
+//
+// User code should not inspect this macro.
+#define ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY 0
+
 #endif  // ABSL_BASE_OPTIONS_H_

absl/hash/internal/hash.h

diff --git a/absl/hash/internal/hash.h b/absl/hash/internal/hash.h
index ae3d9a2..760dd8c 100644
--- a/absl/hash/internal/hash.h
+++ b/absl/hash/internal/hash.h
@@ -79,6 +79,7 @@
 #include "absl/base/internal/endian.h"
 #include "absl/base/internal/unaligned_access.h"
 #include "absl/base/optimization.h"
+#include "absl/base/options.h"
 #include "absl/base/port.h"
 #include "absl/container/fixed_array.h"
 #include "absl/hash/internal/city.h"
@@ -96,6 +97,10 @@
 #include <filesystem>  // NOLINT
 #endif
 
+// We are allowed to use a non-portable hardware-accelerated implementation in
+// headers if ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY != 0
+#if ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY != 0
+
 // 32-bit builds with SSE 4.2 do not have _mm_crc32_u64, so the
 // __x86_64__ condition is necessary.
 #if defined(__SSE4_2__) && defined(__x86_64__)
@@ -129,6 +134,15 @@
 #define ABSL_HASH_INTERNAL_CRC32_U32 __crc32cw
 #define ABSL_HASH_INTERNAL_CRC32_U8 __crc32cb
 
+#endif  // Platform tests
+
+#endif  // ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY != 0
+
+
+#if ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY == 1
+#ifndef ABSL_HASH_INTERNAL_HAS_CRC32
+#error "Hardware acceleration is required by ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY but not supported on this platform; see absl/base/options.h"
+#endif
 #endif
 
 namespace absl {

ci/absl_alternate_options.h

diff --git a/ci/absl_alternate_options.h b/ci/absl_alternate_options.h
index 7f70aad..54a1b78 100644
--- a/ci/absl_alternate_options.h
+++ b/ci/absl_alternate_options.h
@@ -25,5 +25,6 @@
 #define ABSL_OPTION_USE_INLINE_NAMESPACE 1
 #define ABSL_OPTION_INLINE_NAMESPACE_NAME ns
 #define ABSL_OPTION_HARDENED 1
+#define ABSL_OPTION_INLINE_HW_ACCEL_STRATEGY 2
 
 #endif  // ABSL_CI_ABSL_ALTERNATE_OPTIONS_H_