gh-109039: Branch prediction for Tier 2 interpreter (#109038)

This adds a 16-bit inline cache entry to the conditional branch instructions POP_JUMP_IF_{FALSE,TRUE,NONE,NOT_NONE} and their instrumented variants, which is used to keep track of the branch direction.

Each time we encounter these instructions we shift the cache entry left by one and set the bottom bit to whether we jumped.

Then when it's time to translate such a branch to Tier 2 uops, we use the bit count from the cache entry to decided whether to continue translating the "didn't jump" branch or the "jumped" branch.

The counter is initialized to a pattern of alternating ones and zeros to avoid bias.

The .pyc file magic number is updated. There's a new test, some fixes for existing tests, and a few miscellaneous cleanups.

1 files changed, 21 insertions, 7 deletions

diff --git a/Python/optimizer.c b/Python/optimizer.c
index c6d0f9e5bf2..8eca37a7e4a 100644
--- a/Python/optimizer.c
+++ b/Python/optimizer.c
@@ -1,6 +1,7 @@
 #include "Python.h"
 #include "opcode.h"
 #include "pycore_interp.h"
+#include "pycore_bitutils.h"        // _Py_popcount32()
 #include "pycore_opcode_metadata.h" // _PyOpcode_OpName()
 #include "pycore_opcode_utils.h"  // MAX_REAL_OPCODE
 #include "pycore_optimizer.h"     // _Py_uop_analyze_and_optimize()
@@ -501,7 +502,7 @@ translate_bytecode_to_trace(
             code->co_firstlineno,
             2 * INSTR_IP(initial_instr, code));
 
-top:  // Jump here after _PUSH_FRAME
+top:  // Jump here after _PUSH_FRAME or likely branches
     for (;;) {
         RESERVE_RAW(2, "epilogue");  // Always need space for SAVE_IP and EXIT_TRACE
         ADD_TO_TRACE(SAVE_IP, INSTR_IP(instr, code), 0);
@@ -547,16 +548,29 @@ top:  // Jump here after _PUSH_FRAME
             case POP_JUMP_IF_TRUE:
             {
 pop_jump_if_bool:
-                // Assume jump unlikely (TODO: handle jump likely case)
                 RESERVE(1, 2);
-                _Py_CODEUNIT *target_instr =
-                    instr + 1 + _PyOpcode_Caches[_PyOpcode_Deopt[opcode]] + oparg;
                 max_length -= 2;  // Really the start of the stubs
-                uint32_t uopcode = opcode == POP_JUMP_IF_TRUE ?
+                int counter = instr[1].cache;
+                int bitcount = _Py_popcount32(counter);
+                bool jump_likely = bitcount > 8;
+                bool jump_sense = opcode == POP_JUMP_IF_TRUE;
+                uint32_t uopcode = jump_sense ^ jump_likely ?
                     _POP_JUMP_IF_TRUE : _POP_JUMP_IF_FALSE;
+                _Py_CODEUNIT *next_instr = instr + 1 + _PyOpcode_Caches[_PyOpcode_Deopt[opcode]];
+                _Py_CODEUNIT *target_instr = next_instr + oparg;
+                _Py_CODEUNIT *stub_target = jump_likely ? next_instr : target_instr;
+                DPRINTF(4, "%s(%d): counter=%x, bitcount=%d, likely=%d, sense=%d, uopcode=%s\n",
+                        uop_name(opcode), oparg,
+                        counter, bitcount, jump_likely, jump_sense, uop_name(uopcode));
                 ADD_TO_TRACE(uopcode, max_length, 0);
-                ADD_TO_STUB(max_length, SAVE_IP, INSTR_IP(target_instr, code), 0);
+                ADD_TO_STUB(max_length, SAVE_IP, INSTR_IP(stub_target, code), 0);
                 ADD_TO_STUB(max_length + 1, EXIT_TRACE, 0, 0);
+                if (jump_likely) {
+                    DPRINTF(2, "Jump likely (%x = %d bits), continue at byte offset %d\n",
+                            instr[1].cache, bitcount, 2 * INSTR_IP(target_instr, code));
+                    instr = target_instr;
+                    goto top;
+                }
                 break;
             }
 
@@ -927,6 +941,6 @@ PyUnstable_Optimizer_NewUOpOptimizer(void)
     opt->resume_threshold = UINT16_MAX;
     // Need at least 3 iterations to settle specializations.
     // A few lower bits of the counter are reserved for other flags.
-    opt->backedge_threshold = 3 << OPTIMIZER_BITS_IN_COUNTER;
+    opt->backedge_threshold = 16 << OPTIMIZER_BITS_IN_COUNTER;
     return (PyObject *)opt;
 }