From a83fdf2563aad794f0b0a78e534313bbe050d1eb Mon Sep 17 00:00:00 2001
From: Brandt Bucher <brandtbucher@microsoft.com>
Date: Wed, 14 Sep 2022 17:05:04 -0700
Subject: GH-90997: Improve inline cache performance for MSVC (GH-96781)

---
 Include/internal/pycore_code.h                     | 93 +++++-----------------
 .../2022-09-12-15-15-04.gh-issue-90997.sZO8c9.rst  |  2 +
 2 files changed, 21 insertions(+), 74 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2022-09-12-15-15-04.gh-issue-90997.sZO8c9.rst

diff --git a/Include/internal/pycore_code.h b/Include/internal/pycore_code.h
index 7d5fe94..bf59454 100644
--- a/Include/internal/pycore_code.h
+++ b/Include/internal/pycore_code.h
@@ -285,110 +285,55 @@ PyAPI_FUNC(PyObject*) _Py_GetSpecializationStats(void);
 #define EVAL_CALL_STAT_INC_IF_FUNCTION(name, callable) ((void)0)
 #endif  // !Py_STATS
 
-// Cache values are only valid in memory, so use native endianness.
-#ifdef WORDS_BIGENDIAN
+// Utility functions for reading/writing 32/64-bit values in the inline caches.
+// Great care should be taken to ensure that these functions remain correct and
+// performant! They should compile to just "move" instructions on all supported
+// compilers and platforms.
+
+// We use memcpy to let the C compiler handle unaligned accesses and endianness
+// issues for us. It also seems to produce better code than manual copying for
+// most compilers (see https://blog.regehr.org/archives/959 for more info).
 
 static inline void
 write_u32(uint16_t *p, uint32_t val)
 {
-    p[0] = (uint16_t)(val >> 16);
-    p[1] = (uint16_t)(val >>  0);
+    memcpy(p, &val, sizeof(val));
 }
 
 static inline void
 write_u64(uint16_t *p, uint64_t val)
 {
-    p[0] = (uint16_t)(val >> 48);
-    p[1] = (uint16_t)(val >> 32);
-    p[2] = (uint16_t)(val >> 16);
-    p[3] = (uint16_t)(val >>  0);
-}
-
-static inline uint32_t
-read_u32(uint16_t *p)
-{
-    uint32_t val = 0;
-    val |= (uint32_t)p[0] << 16;
-    val |= (uint32_t)p[1] <<  0;
-    return val;
-}
-
-static inline uint64_t
-read_u64(uint16_t *p)
-{
-    uint64_t val = 0;
-    val |= (uint64_t)p[0] << 48;
-    val |= (uint64_t)p[1] << 32;
-    val |= (uint64_t)p[2] << 16;
-    val |= (uint64_t)p[3] <<  0;
-    return val;
-}
-
-#else
-
-static inline void
-write_u32(uint16_t *p, uint32_t val)
-{
-    p[0] = (uint16_t)(val >>  0);
-    p[1] = (uint16_t)(val >> 16);
+    memcpy(p, &val, sizeof(val));
 }
 
 static inline void
-write_u64(uint16_t *p, uint64_t val)
+write_obj(uint16_t *p, PyObject *val)
 {
-    p[0] = (uint16_t)(val >>  0);
-    p[1] = (uint16_t)(val >> 16);
-    p[2] = (uint16_t)(val >> 32);
-    p[3] = (uint16_t)(val >> 48);
+    memcpy(p, &val, sizeof(val));
 }
 
 static inline uint32_t
 read_u32(uint16_t *p)
 {
-    uint32_t val = 0;
-    val |= (uint32_t)p[0] <<  0;
-    val |= (uint32_t)p[1] << 16;
+    uint32_t val;
+    memcpy(&val, p, sizeof(val));
     return val;
 }
 
 static inline uint64_t
 read_u64(uint16_t *p)
 {
-    uint64_t val = 0;
-    val |= (uint64_t)p[0] <<  0;
-    val |= (uint64_t)p[1] << 16;
-    val |= (uint64_t)p[2] << 32;
-    val |= (uint64_t)p[3] << 48;
+    uint64_t val;
+    memcpy(&val, p, sizeof(val));
     return val;
 }
 
-#endif
-
-static inline void
-write_obj(uint16_t *p, PyObject *obj)
-{
-    uintptr_t val = (uintptr_t)obj;
-#if SIZEOF_VOID_P == 8
-    write_u64(p, val);
-#elif SIZEOF_VOID_P == 4
-    write_u32(p, val);
-#else
-    #error "SIZEOF_VOID_P must be 4 or 8"
-#endif
-}
-
 static inline PyObject *
 read_obj(uint16_t *p)
 {
-    uintptr_t val;
-#if SIZEOF_VOID_P == 8
-    val = read_u64(p);
-#elif SIZEOF_VOID_P == 4
-    val = read_u32(p);
-#else
-    #error "SIZEOF_VOID_P must be 4 or 8"
-#endif
-    return (PyObject *)val;
+    PyObject *val;
+    memcpy(&val, p, sizeof(val));
+    return val;
 }
 
 /* See Objects/exception_handling_notes.txt for details.
diff --git a/Misc/NEWS.d/next/Core and Builtins/2022-09-12-15-15-04.gh-issue-90997.sZO8c9.rst b/Misc/NEWS.d/next/Core and Builtins/2022-09-12-15-15-04.gh-issue-90997.sZO8c9.rst
new file mode 100644
index 0000000..4a43e2b
--- /dev/null
+++ b/Misc/NEWS.d/next/Core and Builtins/2022-09-12-15-15-04.gh-issue-90997.sZO8c9.rst	
@@ -0,0 +1,2 @@
+Improve the performance of reading and writing inline bytecode caches on
+some platforms.
-- 
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