Pattern-matching on mutable field surprisingly allocates a closure #10641
Description
I suspect the OCaml developers might already be aware of this issue, however, as a user of the language, I found this behaviour quite surprising. The allocation showed up while optimising some code and was found using memtrace. Below is a reduced test case.
type t = {
mutable buffer: Bytes.t;
mutable line_to_byte: int array;
mutable number_of_lines: int;
}
let get_line_start { line_to_byte; _ } i = Array.get line_to_byte iWhich compiles to (using 4.12.0 with flambda -O3):
0000000000015f40 <camlTest__get_line_start_177>:
15f40: 48 83 ec 08 sub rsp,0x8
15f44: 48 8b 58 08 mov rbx,QWORD PTR [rax+0x8]
15f48: 49 83 ef 20 sub r15,0x20
15f4c: 4d 3b 7e 08 cmp r15,QWORD PTR [r14+0x8]
15f50: 72 2d jb 15f7f <camlTest__get_line_start_177+0x3f>
15f52: 49 8d 47 08 lea rax,[r15+0x8]
15f56: 48 c7 40 f8 f7 0c 00 mov QWORD PTR [rax-0x8],0xcf7
15f5d: 00
15f5e: 48 8d 3d 2b 00 00 00 lea rdi,[rip+0x2b] # 15f90 <camlTest__fun_242>
15f65: 48 89 38 mov QWORD PTR [rax],rdi
15f68: 48 bf 05 00 00 00 00 movabs rdi,0x100000000000005
15f6f: 00 00 01
15f72: 48 89 78 08 mov QWORD PTR [rax+0x8],rdi
15f76: 48 89 58 10 mov QWORD PTR [rax+0x10],rbx
15f7a: 48 83 c4 08 add rsp,0x8
15f7e: c3 ret
15f7f: e8 8c c8 01 00 call 32810 <caml_call_gc>
15f84: eb cc jmp 15f52 <camlTest__get_line_start_177+0x12>
15f86: 66 2e 0f 1f 84 00 00 cs nop WORD PTR [rax+rax*1+0x0]
15f8d: 00 00 00
0000000000015f90 <camlTest__fun_242>:
15f90: 48 83 ec 08 sub rsp,0x8
15f94: 48 8b 5b 10 mov rbx,QWORD PTR [rbx+0x10]
15f98: 48 8b 7b f8 mov rdi,QWORD PTR [rbx-0x8]
15f9c: 48 c1 ef 09 shr rdi,0x9
15fa0: 48 39 c7 cmp rdi,rax
15fa3: 76 0a jbe 15faf <camlTest__fun_242+0x1f>
15fa5: 48 8b 44 83 fc mov rax,QWORD PTR [rbx+rax*4-0x4]
15faa: 48 83 c4 08 add rsp,0x8
15fae: c3 ret
15faf: e8 94 cb 01 00 call 32b48 <caml_ml_array_bound_error>
15fb4: 66 66 2e 0f 1f 84 00 data16 cs nop WORD PTR [rax+rax*1+0x0]
15fbb: 00 00 00 00
15fbf: 90 nop
Changing the function to let get_line_start t i = Array.get t.line_to_byte i or making the field immutable fixes the issue.