Cranelift: mark post-legalization trapping blocks as cold (#5367)

Trapping is a rare event.

cranelift/codegen/src/legalizer/mod.rs

@@ -304,6 +304,9 @@ fn expand_cond_trap(
     let new_block_trap = func.dfg.make_block();
     let new_block_resume = func.dfg.make_block();
 
+    // Trapping is a rare event, mark the trapping block as cold.
+    func.layout.set_cold(new_block_trap);
+
     // Replace trap instruction by the inverted condition.
     if trapz {
         func.dfg.replace(inst).brnz(arg, new_block_resume, &[]);

cranelift/filetests/filetests/legalizer/conditional-traps.clif

@@ -0,0 +1,40 @@
+;; Test the legalizations of `trap[n]z`.
+
+test legalizer
+target aarch64
+target x86_64
+target riscv64
+target s390x
+
+function %trapnz(i32) -> i32 {
+block0(v0: i32):
+    trapnz v0, heap_oob
+    return v0
+}
+
+; check:  block0(v0: i32):
+; nextln:     brz v0, block2
+; nextln:     jump block1
+; nextln: 
+; nextln: block1 cold:
+; nextln:     trap heap_oob
+; nextln: 
+; nextln: block2:
+; nextln:     return v0
+
+function %trapz(i32) -> i32 {
+block0(v0: i32):
+    trapz v0, heap_oob
+    return v0
+}
+
+; check:  block0(v0: i32):
+; nextln:     brnz v0, block2
+; nextln:     jump block1
+; nextln: 
+; nextln: block1 cold:
+; nextln:     trap heap_oob
+; nextln: 
+; nextln: block2:
+; nextln:     return v0
+