YJIT: Avoid register allocation conflict with a higher stack_idx (#9143)

YJIT: Avoid register allocation conflict

with a higher stack_idx

bootstraptest/test_yjit.rb

@@ -4273,3 +4273,21 @@ def entry = yield
   Foo.class_eval "def initialize() #{ivars} end"
   Foo.new
 }
+
+assert_equal '0', %q{
+  def spill
+    1.to_i # not inlined
+  end
+
+  def inline(_stack1, _stack2, _stack3, _stack4, _stack5)
+    0 # inlined
+  end
+
+  def entry
+    # RegTemps is 00111110 prior to the #inline call.
+    # Its return value goes to stack_idx=0, which conflicts with stack_idx=5.
+    inline(spill, 2, 3, 4, 5)
+  end
+
+  entry
+}

yjit/src/core.rs

@@ -428,12 +428,12 @@ impl RegTemps {
 
     /// Return true if there's a register that conflicts with a given stack_idx.
     pub fn conflicts_with(&self, stack_idx: u8) -> bool {
-        let mut other_idx = stack_idx as isize - get_option!(num_temp_regs) as isize;
-        while other_idx >= 0 {
-            if self.get(other_idx as u8) {
+        let mut other_idx = stack_idx as usize % get_option!(num_temp_regs);
+        while other_idx < MAX_REG_TEMPS as usize {
+            if stack_idx as usize != other_idx && self.get(other_idx as u8) {
                 return true;
             }
-            other_idx -= get_option!(num_temp_regs) as isize;
+            other_idx += get_option!(num_temp_regs);
         }
         false
     }
@@ -3450,7 +3450,7 @@ mod tests {
             assert_eq!(reg_temps.get(stack_idx), false);
         }
 
-        // Set 0, 2, 7
+        // Set 0, 2, 7 (RegTemps: 10100001)
         reg_temps.set(0, true);
         reg_temps.set(2, true);
         reg_temps.set(3, true);
@@ -3466,6 +3466,17 @@ mod tests {
         assert_eq!(reg_temps.get(5), false);
         assert_eq!(reg_temps.get(6), false);
         assert_eq!(reg_temps.get(7), true);
+
+        // Test conflicts
+        assert_eq!(5, get_option!(num_temp_regs));
+        assert_eq!(reg_temps.conflicts_with(0), false); // already set, but no conflict
+        assert_eq!(reg_temps.conflicts_with(1), false);
+        assert_eq!(reg_temps.conflicts_with(2), true); // already set, and conflicts with 7
+        assert_eq!(reg_temps.conflicts_with(3), false);
+        assert_eq!(reg_temps.conflicts_with(4), false);
+        assert_eq!(reg_temps.conflicts_with(5), true); // not set, and will conflict with 0
+        assert_eq!(reg_temps.conflicts_with(6), false);
+        assert_eq!(reg_temps.conflicts_with(7), true); // already set, and conflicts with 2
     }
 
     #[test]