Mach-O does not support 19 bit branches

Author: markshannon

Python/jit.c

@@ -334,7 +334,6 @@ patch_aarch64_21rx(unsigned char *location, uint64_t value)
     patch_aarch64_21r(location, value);
 }
 
-
 // 21-bit relative branch.
 void
 patch_aarch64_19r(unsigned char *location, uint64_t value)
@@ -350,21 +349,6 @@ patch_aarch64_19r(unsigned char *location, uint64_t value)
     set_bits(loc32, 5, value, 2, 19);
 }
 
-// 16-bit relative branch.
-void
-patch_aarch64_14r(unsigned char *location, uint64_t value)
-{
-    uint32_t *loc32 = (uint32_t *)location;
-    assert(IS_AARCH64_TEST_AND_BRANCH(*loc32));
-    value -= (uintptr_t)location;
-    // Check that we're not out of range of 16 signed bits:
-    assert((int64_t)value >= -(1 << 15));
-    assert((int64_t)value < (1 << 15));
-    // Since instructions are 4-byte aligned, only use 14 bits:
-    assert(get_bits(value, 0, 2) == 0);
-    set_bits(loc32, 5, value, 2, 14);
-}
-
 // 28-bit relative branch.
 void
 patch_aarch64_26r(unsigned char *location, uint64_t value)

Tools/jit/_optimizers.py

@@ -311,8 +311,16 @@ def run(self) -> None:
         self.path.write_text(self._body())
 
 
+# Mach-O does not support the 19 bit branch locations needed for branch reordering
+class OptimizerAArch64_MachO(Optimizer):  # pylint: disable = too-few-public-methods
+    """aarch64-apple-darwin"""
+
+    # https://developer.arm.com/documentation/ddi0602/2025-03/Base-Instructions/B--Branch-
+    _re_jump = re.compile(r"\s*b\s+(?P<target>[\w.]+)")
+
+
 class OptimizerAArch64(Optimizer):  # pylint: disable = too-few-public-methods
-    """aarch64-apple-darwin/aarch64-pc-windows-msvc/aarch64-unknown-linux-gnu"""
+    """aarch64-pc-windows-msvc/aarch64-unknown-linux-gnu"""
 
     _branches = _AARCH64_BRANCHES
     _re_branch = re.compile(

Tools/jit/_targets.py

@@ -565,7 +565,7 @@ def get_target(host: str) -> _COFF32 | _COFF64 | _ELF | _MachO:
     if re.fullmatch(r"aarch64-apple-darwin.*", host):
         host = "aarch64-apple-darwin"
         condition = "defined(__aarch64__) && defined(__APPLE__)"
-        optimizer = _optimizers.OptimizerAArch64
+        optimizer = _optimizers.OptimizerAArch64_MachO
         target = _MachO(host, condition, optimizer=optimizer)
     elif re.fullmatch(r"aarch64-pc-windows-msvc", host):
         host = "aarch64-pc-windows-msvc"