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/meta/src/shared/settings.rs

@@ -237,6 +237,19 @@ pub(crate) fn define() -> SettingGroup {
         true,
     );
 
+    settings.add_bool(
+        "preserve_frame_pointers",
+        "Preserve frame pointers",
+        r#"
+            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.
+        "#,
+        false,
+    );
+
     settings.add_bool(
         "machine_code_cfg_info",
         "Generate CFG metadata for machine code.",
@@ -324,7 +337,7 @@ pub(crate) fn define() -> SettingGroup {
             for the out-of-bounds case, a misspeculation of that conditional
             branch (falsely predicted in-bounds) will select an in-bounds
             index to load on the speculative path.
-            
+
             This option is enabled by default because it is highly
             recommended for secure sandboxing. The embedder should consider
             the security implications carefully before disabling this option.

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

cranelift/filetests/filetests/isa/aarch64/leaf.clif

@@ -0,0 +1,15 @@
+;; Test compilation of leaf functions without preserving frame pointers.
+
+test compile precise-output
+set unwind_info=false
+set preserve_frame_pointers=false
+target aarch64
+
+function %leaf(i64) -> i64 {
+block0(v0: i64):
+    return v0
+}
+
+; block0:
+;   ret
+

cranelift/filetests/filetests/isa/aarch64/leaf_with_preserve_frame_pointers.clif

@@ -0,0 +1,18 @@
+;; Test compilation of leaf functions while preserving frame pointers.
+
+test compile precise-output
+set unwind_info=false
+set preserve_frame_pointers=true
+target aarch64
+
+function %leaf(i64) -> i64 {
+block0(v0: i64):
+    return v0
+}
+
+;   stp fp, lr, [sp, #-16]!
+;   mov fp, sp
+; block0:
+;   ldp fp, lr, [sp], #16
+;   ret
+

cranelift/filetests/filetests/isa/x64/leaf.clif

@@ -0,0 +1,20 @@
+;; Test compilation of leaf functions without preserving frame pointers.
+
+test compile precise-output
+set unwind_info=false
+set preserve_frame_pointers=false
+target x86_64
+
+function %leaf(i64) -> i64 {
+block0(v0: i64):
+    return v0
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movq    %rdi, %rax
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+

cranelift/filetests/filetests/isa/x64/leaf_with_preserve_frame_pointers.clif

@@ -0,0 +1,20 @@
+;; Test compilation of leaf functions while preserving frame pointers.
+
+test compile precise-output
+set unwind_info=false
+set preserve_frame_pointers=true
+target x86_64
+
+function %leaf(i64) -> i64 {
+block0(v0: i64):
+    return v0
+}
+
+;   pushq   %rbp
+;   movq    %rsp, %rbp
+; block0:
+;   movq    %rdi, %rax
+;   movq    %rbp, %rsp
+;   popq    %rbp
+;   ret
+

crates/wasmtime/src/engine.rs

@@ -393,6 +393,7 @@ impl Engine {
             | "machine_code_cfg_info"
             | "tls_model" // wasmtime doesn't use tls right now
             | "opt_level" // opt level doesn't change semantics
+            | "preserve_frame_pointers" // we don't currently rely on frame pointers
             | "enable_alias_analysis" // alias analysis-based opts don't change semantics
             | "probestack_func_adjusts_sp" // probestack above asserted disabled
             | "probestack_size_log2" // probestack above asserted disabled