Revert "Allow merging bundles that have a fixed-reg def (#155)" (#160)

This reverts commit 1f08342.

Author: elliottt

src/ion/merge.rs

@@ -14,8 +14,7 @@
 
 use super::{Env, LiveBundleIndex, SpillSet, SpillSlotIndex, VRegIndex};
 use crate::{
-    ion::data_structures::{BlockparamOut, CodeRange},
-    Function, Inst, OperandConstraint, OperandKind, PReg, ProgPoint,
+    ion::data_structures::BlockparamOut, Function, Inst, OperandConstraint, OperandKind, PReg,
 };
 use alloc::format;
 use smallvec::smallvec;
@@ -59,21 +58,6 @@ impl<'a, F: Function> Env<'a, F> {
             }
         }
 
-        // If a bundle has a fixed-reg def then we need to be careful to not
-        // extend the bundle to include another use in the same instruction.
-        // This could result in a minimal bundle that is impossible to split.
-        //
-        // This can only happen with an early use and a late def, so we round
-        // the start of each range containing a fixed def up to the start of
-        // its instruction to detect overlaps.
-        let adjust_range_start = |bundle_idx, range: CodeRange| {
-            if self.bundles[bundle_idx].cached_fixed_def() {
-                ProgPoint::before(range.from.inst())
-            } else {
-                range.from
-            }
-        };
-
         // Check for overlap in LiveRanges and for conflicting
         // requirements.
         let ranges_from = &self.bundles[from].ranges[..];
@@ -92,11 +76,9 @@ impl<'a, F: Function> Env<'a, F> {
                 return false;
             }
 
-            if adjust_range_start(from, ranges_from[idx_from].range) >= ranges_to[idx_to].range.to {
+            if ranges_from[idx_from].range.from >= ranges_to[idx_to].range.to {
                 idx_to += 1;
-            } else if adjust_range_start(to, ranges_to[idx_to].range)
-                >= ranges_from[idx_from].range.to
-            {
+            } else if ranges_to[idx_to].range.from >= ranges_from[idx_from].range.to {
                 idx_from += 1;
             } else {
                 // Overlap -- cannot merge.
@@ -109,6 +91,15 @@ impl<'a, F: Function> Env<'a, F> {
             }
         }
 
+        // Avoid merging if either side has a fixed-reg def: this can
+        // result in an impossible-to-solve allocation problem if
+        // there is a fixed-reg use in the same reg on the same
+        // instruction.
+        if self.bundles[from].cached_fixed_def() || self.bundles[to].cached_fixed_def() {
+            trace!(" -> one bundle has a fixed def; aborting merge");
+            return false;
+        }
+
         // Check for a requirements conflict.
         if self.bundles[from].cached_stack()
             || self.bundles[from].cached_fixed()