Cranelift: fix regalloc2 integration bug wrt blockparam branch args. (#4042)

Previously, the block successor accumulation and the blockparam branch
arg setup were decoupled. The lowering backend implicitly specified
the order of successor edges via its `MachTerminator` enum on the last
instruction in the block, while the `Lower` toplevel
machine-independent driver set up blockparam branch args in the edge
order seen in CLIF.

In some cases, these orders did not match -- for example, when the
conditional branch depended on an FP condition that was implemented by
swapping taken/not-taken edges and inverting the condition code.

This PR refactors the successor handling to be centralized in `Lower`
rather than flow through the terminator `MachInst`, and adds a
successor block and its blockparam args at the same time, ensuring the
orders match.

cranelift/codegen/src/isa/aarch64/inst/mod.rs

@@ -98,7 +98,6 @@ pub struct CallIndInfo {
 pub struct JTSequenceInfo {
     pub targets: Vec<BranchTarget>,
     pub default_target: BranchTarget,
-    pub targets_for_term: Vec<MachLabel>, // needed for MachTerminator.
 }
 
 fn count_zero_half_words(mut value: u64, num_half_words: u8) -> usize {
@@ -1091,17 +1090,13 @@ impl MachInst for Inst {
         }
     }
 
-    fn is_term<'a>(&'a self) -> MachTerminator<'a> {
+    fn is_term(&self) -> MachTerminator {
         match self {
             &Inst::Ret { .. } | &Inst::EpiloguePlaceholder => MachTerminator::Ret,
-            &Inst::Jump { dest } => MachTerminator::Uncond(dest.as_label().unwrap()),
-            &Inst::CondBr {
-                taken, not_taken, ..
-            } => MachTerminator::Cond(taken.as_label().unwrap(), not_taken.as_label().unwrap()),
-            &Inst::IndirectBr { ref targets, .. } => MachTerminator::Indirect(&targets[..]),
-            &Inst::JTSequence { ref info, .. } => {
-                MachTerminator::Indirect(&info.targets_for_term[..])
-            }
+            &Inst::Jump { .. } => MachTerminator::Uncond,
+            &Inst::CondBr { .. } => MachTerminator::Cond,
+            &Inst::IndirectBr { .. } => MachTerminator::Indirect,
+            &Inst::JTSequence { .. } => MachTerminator::Indirect,
             _ => MachTerminator::None,
         }
     }

cranelift/codegen/src/isa/aarch64/lower_inst.rs

@@ -2739,7 +2739,6 @@ pub(crate) fn lower_branch<C: LowerCtx<I = Inst>>(
                     .map(|bix| BranchTarget::Label(*bix))
                     .collect();
                 let default_target = BranchTarget::Label(targets[0]);
-                let targets_for_term: Vec<MachLabel> = targets.to_vec();
                 ctx.emit(Inst::JTSequence {
                     ridx,
                     rtmp1,
@@ -2747,7 +2746,6 @@ pub(crate) fn lower_branch<C: LowerCtx<I = Inst>>(
                     info: Box::new(JTSequenceInfo {
                         targets: jt_targets,
                         default_target,
-                        targets_for_term,
                     }),
                 });
             }

cranelift/codegen/src/isa/s390x/inst/mod.rs

@@ -733,19 +733,17 @@ impl MachInst for Inst {
         }
     }
 
-    fn is_term<'a>(&'a self) -> MachTerminator<'a> {
+    fn is_term(&self) -> MachTerminator {
         match self {
             &Inst::Ret { .. } | &Inst::EpiloguePlaceholder => MachTerminator::Ret,
-            &Inst::Jump { dest } => MachTerminator::Uncond(dest),
-            &Inst::CondBr {
-                taken, not_taken, ..
-            } => MachTerminator::Cond(taken, not_taken),
+            &Inst::Jump { .. } => MachTerminator::Uncond,
+            &Inst::CondBr { .. } => MachTerminator::Cond,
             &Inst::OneWayCondBr { .. } => {
                 // Explicitly invisible to CFG processing.
                 MachTerminator::None
             }
-            &Inst::IndirectBr { ref targets, .. } => MachTerminator::Indirect(&targets[..]),
-            &Inst::JTSequence { ref targets, .. } => MachTerminator::Indirect(&targets[..]),
+            &Inst::IndirectBr { .. } => MachTerminator::Indirect,
+            &Inst::JTSequence { .. } => MachTerminator::Indirect,
             _ => MachTerminator::None,
         }
     }

cranelift/codegen/src/isa/x64/inst/mod.rs

@@ -2119,18 +2119,13 @@ impl MachInst for Inst {
         }
     }
 
-    fn is_term<'a>(&'a self) -> MachTerminator<'a> {
+    fn is_term(&self) -> MachTerminator {
         match self {
             // Interesting cases.
             &Self::Ret { .. } | &Self::EpiloguePlaceholder => MachTerminator::Ret,
-            &Self::JmpKnown { dst } => MachTerminator::Uncond(dst),
-            &Self::JmpCond {
-                taken, not_taken, ..
-            } => MachTerminator::Cond(taken, not_taken),
-            &Self::JmpTableSeq {
-                ref targets_for_term,
-                ..
-            } => MachTerminator::Indirect(&targets_for_term[..]),
+            &Self::JmpKnown { .. } => MachTerminator::Uncond,
+            &Self::JmpCond { .. } => MachTerminator::Cond,
+            &Self::JmpTableSeq { .. } => MachTerminator::Indirect,
             // All other cases are boring.
             _ => MachTerminator::None,
         }