Assembly for stg_enter_info starts with a dead 'mov'

In the course of my investigation in #14675 (closed), I happened to look at the x86_64 assembly for stg_enter_info, through gdb:

Breakpoint 2, stg_enter_info () at rts/HeapStackCheck.cmm:166
166	{
(gdb) disassemble 
Dump of assembler code for function stg_enter_info:
=> 0x0000000002cf5900 <+0>:	mov    0x0(%rbp),%rax
   0x0000000002cf5904 <+4>:	mov    0x8(%rbp),%rax
   0x0000000002cf5908 <+8>:	test   $0x7,%al
   0x0000000002cf590a <+10>:	jne    0x2cf593d <stg_enter_info+61>
   0x0000000002cf590c <+12>:	mov    (%rax),%rbx
   0x0000000002cf590f <+15>:	cmpl   $0x1a,-0x8(%rbx)
   0x0000000002cf5913 <+19>:	jb     0x2cf5947 <stg_enter_info+71>
   0x0000000002cf5915 <+21>:	cmpl   $0x1c,-0x8(%rbx)
   0x0000000002cf5919 <+25>:	jb     0x2cf592d <stg_enter_info+45>
   0x0000000002cf591b <+27>:	cmpl   $0x1d,-0x8(%rbx)
   0x0000000002cf591f <+31>:	jb     0x2cf5933 <stg_enter_info+51>
   0x0000000002cf5921 <+33>:	mov    %rbx,%rcx
   0x0000000002cf5924 <+36>:	mov    %rax,%rbx
   0x0000000002cf5927 <+39>:	add    $0x10,%rbp
   0x0000000002cf592b <+43>:	jmpq   *%rcx
   0x0000000002cf592d <+45>:	cmpl   $0x1b,-0x8(%rbx)
   0x0000000002cf5931 <+49>:	jb     0x2cf5921 <stg_enter_info+33>
   0x0000000002cf5933 <+51>:	mov    0x8(%rax),%rax
   0x0000000002cf5937 <+55>:	mov    %rax,0x8(%rbp)
   0x0000000002cf593b <+59>:	jmp    0x2cf5908 <stg_enter_info+8>
   0x0000000002cf593d <+61>:	mov    %rax,%rbx
   0x0000000002cf5940 <+64>:	add    $0x10,%rbp
   0x0000000002cf5944 <+68>:	jmpq   *0x0(%rbp)
   0x0000000002cf5947 <+71>:	cmpl   $0xf,-0x8(%rbx)
   0x0000000002cf594b <+75>:	jb     0x2cf5969 <stg_enter_info+105>
   0x0000000002cf594d <+77>:	cmpl   $0x19,-0x8(%rbx)
   0x0000000002cf5951 <+81>:	jb     0x2cf595d <stg_enter_info+93>
   0x0000000002cf5953 <+83>:	mov    %rax,%rbx
   0x0000000002cf5956 <+86>:	add    $0x10,%rbp
   0x0000000002cf595a <+90>:	jmpq   *0x0(%rbp)
   0x0000000002cf595d <+93>:	movslq -0x8(%rbx),%rcx
   0x0000000002cf5961 <+97>:	cmp    $0x17,%rcx
   0x0000000002cf5965 <+101>:	jne    0x2cf5921 <stg_enter_info+33>
   0x0000000002cf5967 <+103>:	jmp    0x2cf5953 <stg_enter_info+83>
   0x0000000002cf5969 <+105>:	cmpl   $0x8,-0x8(%rbx)
   0x0000000002cf596d <+109>:	jb     0x2cf5921 <stg_enter_info+33>
   0x0000000002cf596f <+111>:	movslq -0x8(%rbx),%rbx
   0x0000000002cf5973 <+115>:	add    $0xfffffffffffffff8,%rbx
   0x0000000002cf5977 <+119>:	jmpq   *0x2e36d90(,%rbx,8)

Notice the first two instructions:

mov    0x0(%rbp),%rax
mov    0x8(%rbp),%rax

We successively write two different things to rax, making the first mov effectively useless. I'm not quite sure the impact is of any significance but it still seems worth writing this down and addressing it at some point in the future, as there's just no way that first instruction will ever be useful.

I think the assembly is generated from this code, which in turns call ENTER, which in turns calls LOAD_INFO, which appears to rcorrespond to the first few instructions in the assembly I pasted, if we ignore the extra mov.

(Assembly generated by ghc 8.4.1 built from the same commit as the alpha1 for 8.4.1 from hvr's PPA, in a 64bits Ubuntu VM. Not sure we generate a dead mov with the other codegens, haven't tested.)

Edited 6 years ago

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Assembly for stg_enter_info starts with a dead 'mov'

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