AArch64: avoid branches with explicit offsets at lowering stage.

In discussions with @bnjbvr, it came up that generating `OneWayCondBr`s
with explicit, hardcoded PC-offsets as part of lowered instruction
sequences is actually unsafe, because the register allocator *might*
insert a spill or reload into the middle of our sequence. We were
careful about this in some cases but somehow missed that it was a
general restriction. Conceptually, all inter-instruction references
should be via labels at the VCode level; explicit offsets are only ever
known at emission time, and resolved by the `MachBuffer`.

To allow for conditional trap checks without modifying the CFG (as seen
by regalloc) during lowering, this PR instead adds a `TrapIf`
pseudo-instruction that conditionally skips a single embedded trap
instruction. It lowers to the same `condbr label ; trap ; label: ...`
sequence, but without the hardcoded branch-target offset in the lowering
code.

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

@@ -2505,149 +2505,148 @@ fn test_aarch64_binemit() {
     ));
 
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Zero(xreg(8)),
-        },
-        "080200B4",
-        "cbz x8, 64",
-    ));
-    insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::NotZero(xreg(8)),
         },
-        "080200B5",
-        "cbnz x8, 64",
+        "480000B40000A0D4",
+        "cbz x8, 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Eq),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Zero(xreg(8)),
         },
-        "00020054",
-        "b.eq 64",
+        "480000B50000A0D4",
+        "cbnz x8, 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Ne),
         },
-        "01020054",
-        "b.ne 64",
+        "400000540000A0D4",
+        "b.eq 8 ; udf",
     ));
-
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Hs),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Eq),
         },
-        "02020054",
-        "b.hs 64",
+        "410000540000A0D4",
+        "b.ne 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Lo),
         },
-        "03020054",
-        "b.lo 64",
+        "420000540000A0D4",
+        "b.hs 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Mi),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Hs),
         },
-        "04020054",
-        "b.mi 64",
+        "430000540000A0D4",
+        "b.lo 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Pl),
         },
-        "05020054",
-        "b.pl 64",
+        "440000540000A0D4",
+        "b.mi 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Vs),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Mi),
         },
-        "06020054",
-        "b.vs 64",
+        "450000540000A0D4",
+        "b.pl 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Vc),
         },
-        "07020054",
-        "b.vc 64",
+        "460000540000A0D4",
+        "b.vs 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Hi),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Vs),
         },
-        "08020054",
-        "b.hi 64",
+        "470000540000A0D4",
+        "b.vc 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Ls),
         },
-        "09020054",
-        "b.ls 64",
+        "480000540000A0D4",
+        "b.hi 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Ge),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Hi),
         },
-        "0A020054",
-        "b.ge 64",
+        "490000540000A0D4",
+        "b.ls 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Lt),
         },
-        "0B020054",
-        "b.lt 64",
+        "4A0000540000A0D4",
+        "b.ge 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Gt),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Ge),
         },
-        "0C020054",
-        "b.gt 64",
+        "4B0000540000A0D4",
+        "b.lt 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Le),
         },
-        "0D020054",
-        "b.le 64",
+        "4C0000540000A0D4",
+        "b.gt 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
-            kind: CondBrKind::Cond(Cond::Al),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Gt),
         },
-        "0E020054",
-        "b.al 64",
+        "4D0000540000A0D4",
+        "b.le 8 ; udf",
     ));
     insns.push((
-        Inst::OneWayCondBr {
-            target: BranchTarget::ResolvedOffset(64),
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
             kind: CondBrKind::Cond(Cond::Nv),
         },
-        "0F020054",
-        "b.nv 64",
+        "4E0000540000A0D4",
+        "b.al 8 ; udf",
+    ));
+    insns.push((
+        Inst::TrapIf {
+            trap_info: (SourceLoc::default(), TrapCode::Interrupt),
+            kind: CondBrKind::Cond(Cond::Al),
+        },
+        "4F0000540000A0D4",
+        "b.nv 8 ; udf",
     ));
 
     insns.push((