bpo-44187: Quickening infrastructure (GH-26264)

* Add co_firstinstr field to code object.

* Implement barebones quickening.

* Use non-quickened bytecode when tracing.

* Add NEWS item

* Add new file to Windows build.

* Don't specialize instructions with EXTENDED_ARG.

Include/internal/pycore_code.h

@@ -4,6 +4,7 @@
 extern "C" {
 #endif
 
+/* Legacy Opcache */
 
 typedef struct {
     PyObject *ptr;  /* Cached pointer (borrowed reference) */
@@ -26,6 +27,129 @@ struct _PyOpcache {
 };
 
 
+/* PEP 659
+ * Specialization and quickening structs and helper functions
+ */
+
+typedef struct {
+    int32_t cache_count;
+    int32_t _; /* Force 8 byte size */
+} _PyEntryZero;
+
+typedef struct {
+    uint8_t original_oparg;
+    uint8_t counter;
+    uint16_t index;
+} _PyAdaptiveEntry;
+
+/* Add specialized versions of entries to this union.
+ *
+ * Do not break the invariant: sizeof(SpecializedCacheEntry) == 8
+ * Preserving this invariant is necessary because:
+    - If any one form uses more space, then all must and on 64 bit machines
+      this is likely to double the memory consumption of caches
+    - The function for calculating the offset of caches assumes a 4:1
+      cache:instruction size ratio. Changing that would need careful
+      analysis to choose a new function.
+ */
+typedef union {
+    _PyEntryZero zero;
+    _PyAdaptiveEntry adaptive;
+} SpecializedCacheEntry;
+
+#define INSTRUCTIONS_PER_ENTRY (sizeof(SpecializedCacheEntry)/sizeof(_Py_CODEUNIT))
+
+/* Maximum size of code to quicken, in code units. */
+#define MAX_SIZE_TO_QUICKEN 5000
+
+typedef union _cache_or_instruction {
+    _Py_CODEUNIT code[1];
+    SpecializedCacheEntry entry;
+} SpecializedCacheOrInstruction;
+
+/* Get pointer to the nth cache entry, from the first instruction and n.
+ * Cache entries are indexed backwards, with [count-1] first in memory, and [0] last.
+ * The zeroth entry immediately precedes the instructions.
+ */
+static inline SpecializedCacheEntry *
+_GetSpecializedCacheEntry(_Py_CODEUNIT *first_instr, Py_ssize_t n)
+{
+    SpecializedCacheOrInstruction *last_cache_plus_one = (SpecializedCacheOrInstruction *)first_instr;
+    assert(&last_cache_plus_one->code[0] == first_instr);
+    return &last_cache_plus_one[-1-n].entry;
+}
+
+/* Following two functions form a pair.
+ *
+ * oparg_from_offset_and_index() is used to compute the oparg
+ * when quickening, so that offset_from_oparg_and_nexti()
+ * can be used at runtime to compute the offset.
+ *
+ * The relationship between the three values is currently
+ *     offset == (index>>1) + oparg
+ * This relation is chosen based on the following observations:
+ * 1. typically 1 in 4 instructions need a cache
+ * 2. instructions that need a cache typically use 2 entries
+ *  These observations imply:  offset ≈ index/2
+ *  We use the oparg to fine tune the relation to avoid wasting space
+ * and allow consecutive instructions to use caches.
+ *
+ * If the number of cache entries < number of instructions/2 we will waste
+ * some small amoount of space.
+ * If the number of cache entries > (number of instructions/2) + 255, then
+ * some instructions will not be able to use a cache.
+ * In practice, we expect some small amount of wasted space in a shorter functions
+ * and only functions exceeding a 1000 lines or more not to have enugh cache space.
+ *
+ */
+static inline int
+oparg_from_offset_and_nexti(int offset, int nexti)
+{
+    return offset-(nexti>>1);
+}
+
+static inline int
+offset_from_oparg_and_nexti(int oparg, int nexti)
+{
+    return (nexti>>1)+oparg;
+}
+
+/* Get pointer to the cache entry associated with an instruction.
+ * nexti is the index of the instruction plus one.
+ * nexti is used as it corresponds to the instruction pointer in the interpreter.
+ * This doesn't check that an entry has been allocated for that instruction. */
+static inline SpecializedCacheEntry *
+_GetSpecializedCacheEntryForInstruction(_Py_CODEUNIT *first_instr, int nexti, int oparg)
+{
+    return _GetSpecializedCacheEntry(
+        first_instr,
+        offset_from_oparg_and_nexti(oparg, nexti)
+    );
+}
+
+#define QUICKENING_WARMUP_DELAY 8
+
+/* We want to compare to zero for efficiency, so we offset values accordingly */
+#define QUICKENING_INITIAL_WARMUP_VALUE (-QUICKENING_WARMUP_DELAY)
+#define QUICKENING_WARMUP_COLDEST 1
+
+static inline void
+PyCodeObject_IncrementWarmup(PyCodeObject * co)
+{
+    co->co_warmup++;
+}
+
+/* Used by the interpreter to determine when a code object should be quickened */
+static inline int
+PyCodeObject_IsWarmedUp(PyCodeObject * co)
+{
+    return (co->co_warmup == 0);
+}
+
+int _Py_Quicken(PyCodeObject *code);
+
+extern Py_ssize_t _Py_QuickenedCount;
+
 struct _PyCodeConstructor {
     /* metadata */
     PyObject *filename;