#14753: Assembly for stg_enter_info starts with a dead 'mov'
-------------------------------------+-------------------------------------
Reporter: alpmestan | Owner: (none)
Type: bug | Status: new
Priority: low | Milestone:
Component: Compiler | Version: 8.4.1-alpha1
(CodeGen) |
Keywords: | Operating System: Linux
Architecture: x86_64 | Type of failure: None/Unknown
(amd64) |
Test Case: | Blocked By:
Blocking: | Related Tickets:
Differential Rev(s): | Wiki Page:
-------------------------------------+-------------------------------------
In the course of my investigation in #14675, I happened to look at the
x86_64 assembly for `stg_enter_info`, through gdb:
{{{#!asm
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
0x0000000002cf590c <+12>: mov (%rax),%rbx
0x0000000002cf590f <+15>: cmpl $0x1a,-0x8(%rbx)
0x0000000002cf5913 <+19>: jb 0x2cf5947
0x0000000002cf5915 <+21>: cmpl $0x1c,-0x8(%rbx)
0x0000000002cf5919 <+25>: jb 0x2cf592d
0x0000000002cf591b <+27>: cmpl $0x1d,-0x8(%rbx)
0x0000000002cf591f <+31>: jb 0x2cf5933
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
0x0000000002cf5933 <+51>: mov 0x8(%rax),%rax
0x0000000002cf5937 <+55>: mov %rax,0x8(%rbp)
0x0000000002cf593b <+59>: jmp 0x2cf5908
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
0x0000000002cf594d <+77>: cmpl $0x19,-0x8(%rbx)
0x0000000002cf5951 <+81>: jb 0x2cf595d
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
0x0000000002cf5967 <+103>: jmp 0x2cf5953
0x0000000002cf5969 <+105>: cmpl $0x8,-0x8(%rbx)
0x0000000002cf596d <+109>: jb 0x2cf5921
0x0000000002cf596f <+111>: movslq -0x8(%rbx),%rbx
0x0000000002cf5973 <+115>: add $0xfffffffffffffff8,%rbx
0x0000000002cf5977 <+119>: jmpq *0x2e36d90(,%rbx,8)
}}}
Notice the first two instructions:
{{{#!asm
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
[https://github.com/ghc/ghc/blob/master/rts/HeapStackCheck.cmm#L164 this
code], which in turns call
[https://github.com/ghc/ghc/blob/master/includes/Cmm.h#L301 `ENTER`],
which in turns calls
[https://github.com/ghc/ghc/blob/master/includes/Cmm.h#L272 `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.)
--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14753
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler
