Save/restore callee-saved registers used in regmove/regfill.

The regmove and regfill instructions temporarily divert a value's
location, and these temporary diversions are not reflected in
`func.locations`. For now, make an extra scan through the instructions
of the function to find any regmove or regfill instructions in order to
find all used callee-saved registers.

This fixes #296.

cranelift/filetests/isa/intel/prologue-epilogue.cton

@@ -207,3 +207,25 @@ ebb0(v0: i64, v1: i64):
 ; nextln:     v54 = x86_pop.i64
 ; nextln:     return v54, v55, v56, v57, v58, v59
 ; nextln: }
+
+; A function which uses diverted registers.
+
+function %divert(i32) -> i32 system_v {
+ebb0(v0: i32):
+    v2 = iconst.i32 0
+    v3 = iconst.i32 1
+    jump ebb3(v0, v3, v2)
+
+ebb3(v4: i32, v5: i32, v6: i32):
+    brz v4, ebb4
+    v7 = iadd v5, v6
+    v8 = iadd_imm v4, -1
+    jump ebb3(v8, v7, v5)
+
+ebb4:
+    return v5
+}
+
+; check: function %divert
+; check: regmove v5, %rcx -> %rbx
+; check: [RexOp1popq#58,%rbx]                v15 = x86_pop.i64

lib/cretonne/src/isa/intel/abi.rs

@@ -184,6 +184,26 @@ fn callee_saved_gprs_used(flags: &shared_settings::Flags, func: &ir::Function) -
         }
     }
 
+    // regmove and regfill instructions may temporarily divert values into other registers,
+    // and these are not reflected in `func.locations`. Scan the function for such instructions
+    // and note which callee-saved registers they use.
+    //
+    // TODO: Consider re-evaluating how regmove/regfill/regspill work and whether it's possible
+    // to avoid this step.
+    for ebb in &func.layout {
+        for inst in func.layout.ebb_insts(ebb) {
+            match func.dfg[inst] {
+                ir::instructions::InstructionData::RegMove { dst, .. } |
+                ir::instructions::InstructionData::RegFill { dst, .. } => {
+                    if !used.is_avail(GPR, dst) {
+                        used.free(GPR, dst);
+                    }
+                }
+                _ => (),
+            }
+        }
+    }
+
     used.intersect(&all_callee_saved);
     return used;
 }