YJIT: Invoke PosMarker callbacks only with solid positions

Previously, PosMarker callbacks ran even when the assembler failed to
assemble its contents due to insufficient space. This was problematic
because when Assembler::compile() failed, the callbacks were given
positions that have no valid code, contrary to general expectation.

For example, we use a PosMarker callback to record VM instruction
boundaries and patch in jumps to exits in case the guest program starts
tracing, however, previously, we could record a location near the end of
the code block, where there is no space to patch in jumps. I suspect
this is the cause of the recent occurrences of rare random failures on
GitHub Actions with the invariants.rs:529 "can rewrite existing code"
message. `--yjit-perf` also uses PosMarker and had a similar issue.

Buffer the list of callbacks to fire, and only fire them when all code
in the assembler are written out successfully. It's more intuitive this
way.

yjit/src/backend/arm64/mod.rs

@@ -819,6 +819,9 @@ impl Assembler
         // List of GC offsets
         let mut gc_offsets: Vec<u32> = Vec::new();
 
+        // Buffered list of PosMarker callbacks to fire if codegen is successful
+        let mut pos_markers: Vec<(usize, CodePtr)> = vec![];
+
         // For each instruction
         let start_write_pos = cb.get_write_pos();
         let mut insn_idx: usize = 0;
@@ -838,8 +841,8 @@ impl Assembler
                     cb.write_label(target.unwrap_label_idx());
                 },
                 // Report back the current position in the generated code
-                Insn::PosMarker(pos_marker) => {
-                    pos_marker(cb.get_write_ptr());
+                Insn::PosMarker(..) => {
+                    pos_markers.push((insn_idx, cb.get_write_ptr()))
                 }
                 Insn::BakeString(text) => {
                     for byte in text.as_bytes() {
@@ -1205,7 +1208,21 @@ impl Assembler
             }
         }
 
-        Ok(gc_offsets)
+        // Error if we couldn't write out everything
+        if cb.has_dropped_bytes() {
+            return Err(EmitError::OutOfMemory)
+        } else {
+            // No bytes dropped, so the pos markers point to valid code
+            for (insn_idx, pos) in pos_markers {
+                if let Insn::PosMarker(callback) = self.insns.get(insn_idx).unwrap() {
+                    callback(pos);
+                } else {
+                    panic!("non-PosMarker in pos_markers insn_idx={insn_idx} {self:?}");
+                }
+            }
+
+            return Ok(gc_offsets)
+        }
     }
 
     /// Optimize and compile the stored instructions

yjit/src/backend/tests.rs

@@ -310,3 +310,21 @@ fn test_cmp_8_bit() {
 
     asm.compile_with_num_regs(&mut cb, 1);
 }
+
+#[test]
+fn test_no_pos_marker_callback_when_compile_fails() {
+    // When compilation fails (e.g. when out of memory), the code written out is malformed.
+    // We don't want to invoke the pos_marker callbacks with positions of malformed code.
+    let mut asm = Assembler::new();
+
+    // Markers around code to exhaust memory limit
+    let fail_if_called = |_code_ptr| panic!("pos_marker callback should not be called");
+    asm.pos_marker(fail_if_called);
+    let zero = asm.load(0.into());
+    let sum = asm.add(zero, 500.into());
+    asm.store(Opnd::mem(64, SP, 8), sum);
+    asm.pos_marker(fail_if_called);
+
+    let cb = &mut CodeBlock::new_dummy(8);
+    assert!(asm.compile(cb, None).is_none(), "should fail due to tiny size limit");
+}

yjit/src/backend/x86_64/mod.rs

@@ -457,6 +457,9 @@ impl Assembler
         // List of GC offsets
         let mut gc_offsets: Vec<u32> = Vec::new();
 
+        // Buffered list of PosMarker callbacks to fire if codegen is successful
+        let mut pos_markers: Vec<(usize, CodePtr)> = vec![];
+
         // For each instruction
         let start_write_pos = cb.get_write_pos();
         let mut insn_idx: usize = 0;
@@ -479,8 +482,8 @@ impl Assembler
                 },
 
                 // Report back the current position in the generated code
-                Insn::PosMarker(pos_marker) => {
-                    pos_marker(cb.get_write_ptr());
+                Insn::PosMarker(..) => {
+                    pos_markers.push((insn_idx, cb.get_write_ptr()));
                 },
 
                 Insn::BakeString(text) => {
@@ -817,7 +820,21 @@ impl Assembler
             }
         }
 
-        Some(gc_offsets)
+        // Error if we couldn't write out everything
+        if cb.has_dropped_bytes() {
+            return None
+        } else {
+            // No bytes dropped, so the pos markers point to valid code
+            for (insn_idx, pos) in pos_markers {
+                if let Insn::PosMarker(callback) = self.insns.get(insn_idx).unwrap() {
+                    callback(pos);
+                } else {
+                    panic!("non-PosMarker in pos_markers insn_idx={insn_idx} {self:?}");
+                }
+            }
+
+            return Some(gc_offsets)
+        }
     }
 
     /// Optimize and compile the stored instructions