cranelift: Add a flag for preserving frame pointers (#4469)

Preserving frame pointers -- even inside leaf functions -- makes it easy to
capture the stack of a running program, without requiring any side tables or
metadata (like `.eh_frame` sections). Many sampling profilers and similar tools
walk frame pointers to capture stacks. Enabling this option will play nice with
those tools.

cranelift/codegen/src/machinst/abi_impl.rs

@@ -1410,12 +1410,13 @@ impl<M: ABIMachineSpec> ABICallee for ABICalleeImpl<M> {
 
         if !self.call_conv.extends_baldrdash() {
             self.fixed_frame_storage_size += total_stacksize;
-            self.setup_frame = M::is_frame_setup_needed(
-                self.is_leaf,
-                self.stack_args_size(),
-                clobbered_callee_saves.len(),
-                self.fixed_frame_storage_size,
-            );
+            self.setup_frame = self.flags.preserve_frame_pointers()
+                || M::is_frame_setup_needed(
+                    self.is_leaf,
+                    self.stack_args_size(),
+                    clobbered_callee_saves.len(),
+                    self.fixed_frame_storage_size,
+                );
 
             insts.extend(
                 M::gen_debug_frame_info(self.call_conv, &self.flags, &self.isa_flags).into_iter(),

cranelift/codegen/src/settings.rs

@@ -541,6 +541,7 @@ enable_atomics = true
 enable_safepoints = false
 enable_llvm_abi_extensions = false
 unwind_info = true
+preserve_frame_pointers = false
 machine_code_cfg_info = false
 emit_all_ones_funcaddrs = false
 enable_probestack = true