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IDProjectCategoryView StatusDate SubmittedLast Update
0006339OCamlback end (clambda to assembly)public2014-03-06 15:422016-12-07 14:03
Assigned Toshinwell 
PrioritynormalSeverityfeatureReproducibilityhave not tried
PlatformOSOS Version
Product Version 
Target VersionFixed in Version4.03.0 
Summary0006339: Useless indirection to access a known top-level function
DescriptionConsider the following piece of code:

let f x = x + 1
let fl l = f l

ocamlopt will compile this structure by putting f and fl into fields of the global symbol corresponding to the current unit, and turn the reference to f (in fl) into a field access of this global symbol. Later in the compilation process (while mapping from lambda to ulambda), the compiler detects that f is actually a constant closure. This information is available (as a Value_closure approximation) when the code for fl is emitted, but there is no way to represent a reference to a known closure in the clambda code. The useless reference (to a statically known value) remains in the generated code as can be seen on the -dcmm or -S output.

The resulting assembly code for fl is:

    movq	%rax, %rbx
    movq	camlTup@GOTPCREL(%rip), %rax
    movq	(%rax), %rax
    jmp	camlList__map_1040@PLT

while it could be:

    movq	%rax, %rbx
    movq	camlTup__3@GOTPCREL(%rip), %rax
    jmp	camlList__map_1040@PLT

Certainly not a big deal, but it should not be too difficult to improve it.

A possible approach would be to treat constant closures (Uclosure(_, [])) as structured constants, allocated during the closure pass.
Attached Filesdiff file icon pr_6339.diff [^] (22,890 bytes) 2014-03-10 19:07 [Show Content]

- Relationships

-  Notes
frisch (developer)
2014-03-10 19:15

Attached a diff, which keeps track of constant closure and allows to refer to them directly, thus avoiding some indirections. Moreover, since it considers constant closures as atomic values, they don't prevent inlining. This means that a function such as:

let foo l = (fun x -> x + 1) l


let foo l =
  let f x = x + 1 in f l

can be inlined. This can give a non-negligible speedup (-20% when calling this function foo in a tight loop on an empty list).

Simply avoiding the indirection does not seem to bring much, even on small examples. It might still be a good idea to do so, since reducing the code size is always a good idea.

The patch also detects more cases of closure which don't need their environment.
shinwell (developer)
2016-12-07 14:03

Flambda should fix these problems.

- Issue History
Date Modified Username Field Change
2014-03-06 15:42 frisch New Issue
2014-03-06 15:48 frisch Description Updated View Revisions
2014-03-06 15:56 frisch Description Updated View Revisions
2014-03-10 19:07 frisch File Added: pr_6339.diff
2014-03-10 19:15 frisch Note Added: 0011036
2014-05-16 20:59 doligez Tag Attached: patch
2014-07-16 15:33 doligez Status new => acknowledged
2016-12-07 14:03 shinwell Note Added: 0016703
2016-12-07 14:03 shinwell Status acknowledged => resolved
2016-12-07 14:03 shinwell Fixed in Version => 4.03.0
2016-12-07 14:03 shinwell Resolution open => fixed
2016-12-07 14:03 shinwell Assigned To => shinwell
2017-02-23 16:35 doligez Category OCaml backend (code generation) => Back end (clambda to assembly)
2017-02-23 16:44 doligez Category Back end (clambda to assembly) => back end (clambda to assembly)

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