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/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
+