Reduce branch precision down to 64 bits

Author: zetanumbers

compiler/rustc_data_structures/src/sync/branch_key.rs

@@ -1,14 +1,14 @@
 use std::cmp;
 
 #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
-pub struct BranchKey(u128);
+pub struct BranchKey(u64);
 
 impl BranchKey {
     pub const fn root() -> Self {
-        Self(0x80000000_00000000_00000000_00000000)
+        Self(0x80000000_00000000)
     }
 
-    fn bits_branch(self, branch_num: u128, bits: u32) -> Result<Self, BranchNestingError> {
+    fn bits_branch(self, branch_num: u64, bits: u32) -> Result<Self, BranchNestingError> {
         let trailing_zeros = self.0.trailing_zeros();
         let allocated_shift = trailing_zeros.checked_sub(bits).ok_or(BranchNestingError(()))?;
         Ok(BranchKey(
@@ -18,7 +18,7 @@ impl BranchKey {
         ))
     }
 
-    pub fn branch(self, branch_num: u128, branch_space: u128) -> BranchKey {
+    pub fn branch(self, branch_num: u64, branch_space: u64) -> BranchKey {
         debug_assert!(
             branch_num < branch_space,
             "branch_num = {branch_num} should be less than branch_space = {branch_space}"
@@ -35,7 +35,7 @@ impl BranchKey {
     pub fn nest(self, then: Self) -> Result<Self, BranchNestingError> {
         let trailing_zeros = then.0.trailing_zeros();
         let branch_num = then.0.wrapping_shr(trailing_zeros + 1);
-        let bits = u128::BITS - trailing_zeros;
+        let bits = u64::BITS - trailing_zeros;
         self.bits_branch(branch_num, bits)
     }
 }

compiler/rustc_middle/src/sync.rs

@@ -56,7 +56,7 @@ macro_rules! parallel {
     }
 
 // This function only works when `is_dyn_thread_safe()`.
-pub fn scope<'scope, OP, R>(spawn_limit: u128, op: OP) -> R
+pub fn scope<'scope, OP, R>(spawn_limit: u64, op: OP) -> R
 where
     OP: for<'a, 'tcx> FnOnce(Scope<'a, 'scope>) -> R + DynSend,
     R: DynSend,
@@ -70,8 +70,8 @@ where
 
 pub struct Scope<'a, 'scope> {
     scope: &'a rustc_thread_pool::Scope<'scope>,
-    branch_limit: u128,
-    next_branch: u128,
+    branch_limit: u64,
+    next_branch: u64,
 }
 
 impl<'a, 'scope> Scope<'a, 'scope> {