Track register pressure for dead EBB parameters.

The spiller wasn't tracking register pressure correctly for dead EBB
parameters in visit_ebb_header(). Make sure we free any dead EBB
parameters.

Fixes #223

cranelift/filetests/regalloc/spill-noregs.cton

@@ -0,0 +1,176 @@
+test regalloc
+set is_64bit
+isa intel
+
+; Test case found by the Binaryen fuzzer.
+;
+; The spiller panics with a
+; 'Ran out of GPR registers when inserting copy before v68 = icmp.i32 eq v66, v67',
+; lib/cretonne/src/regalloc/spilling.rs:425:28 message.
+;
+; The process_reg_uses() function is trying to insert a copy before the icmp instruction in ebb4
+; and runs out of registers to spill. Note that ebb7 has a lot of dead parameter values.
+;
+; The spiller was not releasing register pressure for dead EBB parameters.
+
+function %pr223(i32 [%rdi], i64 vmctx [%rsi]) -> i64 [%rax] native {
+ebb0(v0: i32, v1: i64):
+    v2 = iconst.i32 0
+    v3 = iconst.i64 0
+    v4 = iconst.i32 0xffff_ffff_bb3f_4a2c
+    brz v4, ebb5
+    jump ebb1
+
+ebb1:
+    v5 = iconst.i32 0
+    v6 = copy.i64 v3
+    v7 = copy.i64 v3
+    v8 = copy.i64 v3
+    v9 = copy.i64 v3
+    v10 = copy.i64 v3
+    v11 = copy.i64 v3
+    v12 = copy.i64 v3
+    v13 = copy.i64 v3
+    v14 = copy.i64 v3
+    v15 = copy.i64 v3
+    v16 = copy.i64 v3
+    brnz v5, ebb4(v2, v3, v6, v7, v8, v9, v10, v11, v12, v13, v14, v15, v16)
+    jump ebb2
+
+ebb2:
+    v17 = iconst.i32 0
+    v18 = copy.i64 v3
+    v19 = copy.i64 v3
+    v20 = copy.i64 v3
+    v21 = copy.i64 v3
+    v22 = copy.i64 v3
+    v23 = copy.i64 v3
+    v24 = copy.i64 v3
+    v25 = copy.i64 v3
+    v26 = copy.i64 v3
+    v27 = copy.i64 v3
+    v28 = copy.i64 v3
+    brnz v17, ebb4(v2, v3, v18, v19, v20, v21, v22, v23, v24, v25, v26, v27, v28)
+    jump ebb3
+
+ebb3:
+    jump ebb1
+
+ebb4(v29: i32, v30: i64, v31: i64, v32: i64, v33: i64, v34: i64, v35: i64, v36: i64, v37: i64, v38: i64, v39: i64, v40: i64, v41: i64):
+    jump ebb7(v29, v30, v31, v32, v33, v34, v35, v36, v37, v38, v39, v40, v41)
+
+ebb5:
+    jump ebb6
+
+ebb6:
+    v42 = copy.i64 v3
+    v43 = copy.i64 v3
+    v44 = copy.i64 v3
+    v45 = copy.i64 v3
+    v46 = copy.i64 v3
+    v47 = copy.i64 v3
+    v48 = copy.i64 v3
+    v49 = copy.i64 v3
+    v50 = copy.i64 v3
+    v51 = copy.i64 v3
+    v52 = copy.i64 v3
+    jump ebb7(v2, v3, v42, v43, v44, v45, v46, v47, v48, v49, v50, v51, v52)
+
+ebb7(v53: i32, v54: i64, v55: i64, v56: i64, v57: i64, v58: i64, v59: i64, v60: i64, v61: i64, v62: i64, v63: i64, v64: i64, v65: i64):
+    v66 = iconst.i32 0
+    v67 = iconst.i32 0
+    v68 = icmp eq v66, v67
+    v69 = bint.i32 v68
+    jump ebb8
+
+ebb8:
+    jump ebb9
+
+ebb9:
+    v70 = iconst.i32 0xffff_ffff_ffff_912f
+    brz v70, ebb10
+    jump ebb35
+
+ebb10:
+    v71 = iconst.i32 0
+    brz v71, ebb11
+    jump ebb27
+
+ebb11:
+    jump ebb12
+
+ebb12:
+    jump ebb13
+
+ebb13:
+    jump ebb14
+
+ebb14:
+    jump ebb15
+
+ebb15:
+    jump ebb16
+
+ebb16:
+    jump ebb17
+
+ebb17:
+    jump ebb18
+
+ebb18:
+    jump ebb19
+
+ebb19:
+    jump ebb20
+
+ebb20:
+    jump ebb21
+
+ebb21:
+    jump ebb22
+
+ebb22:
+    jump ebb23
+
+ebb23:
+    jump ebb24
+
+ebb24:
+    jump ebb25
+
+ebb25:
+    jump ebb26
+
+ebb26:
+    jump ebb27
+
+ebb27:
+    jump ebb28
+
+ebb28:
+    jump ebb29
+
+ebb29:
+    jump ebb30
+
+ebb30:
+    jump ebb31
+
+ebb31:
+    jump ebb32
+
+ebb32:
+    jump ebb33
+
+ebb33:
+    jump ebb34
+
+ebb34:
+    jump ebb35
+
+ebb35:
+    jump ebb36
+
+ebb36:
+    trap user0
+}

lib/cretonne/src/regalloc/coloring.rs

@@ -158,7 +158,7 @@ impl<'a> Context<'a> {
     fn visit_ebb(&mut self, ebb: Ebb, tracker: &mut LiveValueTracker) {
         dbg!("Coloring {}:", ebb);
         let mut regs = self.visit_ebb_header(ebb, tracker);
-        tracker.drop_dead_args();
+        tracker.drop_dead_params();
         self.divert.clear();
 
         // Now go through the instructions in `ebb` and color the values they define.

lib/cretonne/src/regalloc/live_value_tracker.rs

@@ -160,9 +160,9 @@ impl LiveValueTracker {
     /// been visited first.
     ///
     /// Returns `(liveins, args)` as a pair of slices. The first slice is the set of live-in values
-    /// from the immediate dominator. The second slice is the set of `ebb` arguments that are live.
+    /// from the immediate dominator. The second slice is the set of `ebb` parameters.
     ///
-    /// Dead arguments with no uses are included in `args`. Call `drop_dead_args()` to remove them.
+    /// Dead parameters with no uses are included in `args`. Call `drop_dead_args()` to remove them.
     pub fn ebb_top(
         &mut self,
         ebb: Ebb,
@@ -314,8 +314,8 @@ impl LiveValueTracker {
 
     /// Drop any values that are marked as `is_dead`.
     ///
-    /// Use this after calling `ebb_top` to clean out dead EBB arguments.
+    /// Use this after calling `ebb_top` to clean out dead EBB parameters.
-    pub fn drop_dead_args(&mut self) {
+    pub fn drop_dead_params(&mut self) {
         self.live.remove_dead_values();
     }
 

lib/cretonne/src/regalloc/reload.rs

@@ -120,7 +120,7 @@ impl<'a> Context<'a> {
     fn visit_ebb(&mut self, ebb: Ebb, tracker: &mut LiveValueTracker) {
         dbg!("Reloading {}:", ebb);
         self.visit_ebb_header(ebb, tracker);
-        tracker.drop_dead_args();
+        tracker.drop_dead_params();
 
         // visit_ebb_header() places us at the first interesting instruction in the EBB.
         while let Some(inst) = self.cur.current_inst() {

lib/cretonne/src/regalloc/spilling.rs

@@ -120,7 +120,8 @@ impl<'a> Context<'a> {
         dbg!("Spilling {}:", ebb);
         self.cur.goto_top(ebb);
         self.visit_ebb_header(ebb, tracker);
-        tracker.drop_dead_args();
+        tracker.drop_dead_params();
+        self.process_spills(tracker);
 
         while let Some(inst) = self.cur.next_inst() {
             if let Some(constraints) =
@@ -163,8 +164,22 @@ impl<'a> Context<'a> {
         }
     }
 
+    // Free all dead registers in `regs` from the pressure set.
+    fn free_dead_regs(&mut self, regs: &[LiveValue]) {
+        for lv in regs {
+            if lv.is_dead {
+                if let Affinity::Reg(rci) = lv.affinity {
+                    if !self.spills.contains(&lv.value) {
+                        let rc = self.reginfo.rc(rci);
+                        self.pressure.free(rc);
+                    }
+                }
+            }
+        }
+    }
+
     fn visit_ebb_header(&mut self, ebb: Ebb, tracker: &mut LiveValueTracker) {
-        let (liveins, args) = tracker.ebb_top(
+        let (liveins, params) = tracker.ebb_top(
             ebb,
             &self.cur.func.dfg,
             self.liveness,
@@ -177,22 +192,17 @@ impl<'a> Context<'a> {
         self.pressure.reset();
         self.take_live_regs(liveins);
 
-        // An EBB can have an arbitrary (up to 2^16...) number of EBB arguments, so they are not
+        // An EBB can have an arbitrary (up to 2^16...) number of parameters, so they are not
         // guaranteed to fit in registers.
-        for lv in args {
+        for lv in params {
             if let Affinity::Reg(rci) = lv.affinity {
                 let rc = self.reginfo.rc(rci);
                 'try_take: while let Err(mask) = self.pressure.take_transient(rc) {
-                    dbg!(
+                    dbg!("Need {} reg for EBB param {}", rc, lv.value);
-                        "Need {} reg for EBB argument {} from {} live-ins",
-                        rc,
-                        lv.value,
-                        liveins.len()
-                    );
                     match self.spill_candidate(mask, liveins) {
                         Some(cand) => {
                             dbg!(
-                                "Spilling live-in {} to make room for {} EBB argument {}",
+                                "Spilling live-in {} to make room for {} EBB param {}",
                                 cand,
                                 rc,
                                 lv.value
@@ -217,6 +227,7 @@ impl<'a> Context<'a> {
 
         // The transient pressure counts for the EBB arguments are accurate. Just preserve them.
         self.pressure.preserve_transient();
+        self.free_dead_regs(params);
     }
 
     fn visit_inst(