Is test(Map = #{}) -> ok. the same as test(Map) when is_map(Map) -> ok.?

Questions / Help
1

If not, which one is more efficient? If yes, would you like to point to the source code that does this optimization? Thanks in advance.

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2 
$ cat <<'EOF' > test.erl
-module(test).

-export([noguard/1, guard/1]).

noguard(_Map = #{}) -> ok.

guard(Map) when is_map(Map) -> ok.
EOF

$ erl
Erlang/OTP 25 [erts-13.1.5] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [jit:ns]

Eshell V13.1.5  (abort with ^G)
1> compile:file(test, ['S']).
{ok,test}
2> q().

$ cat test.S
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3

I think what @jimdigriz was trying to say here is, yes they are equivalent :slight_smile:

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4

Thanks for the replies. I tried the test code using c(test,‘S’) and they are equivalent, using is_map to check. But if I use c(test,to_core) and the output code are different:
test(Map = #{}) → ok.
has Map = ~{}~ in its code. That looks like constructing an empty map then match it to Map.
I am curious where in the compiler source code optimizes this constructing an empty map step into directly calling is_map.

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5

You’re going to want to study : otp/beam_core_to_ssa.erl at master · erlang/otp · GitHub

Edit:

I think one of the lines you’re interested in is going to be here

However, there’s a lot going on and I’ve never studied this code in anger, thus I would ask @bjorng (who also may have the best avatar ever!) to confirm or deny this, and perhaps breakdown the paths within this module for the case you’re interested in.

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6

I don’t think there is any significant difference:

./erlperf 'test:guard(#{}).' 'test:noguard(#{}).' -s 100 -d 100
Code                   ||   Samples       Avg   StdDev    Median      P99  Iteration    Rel
test:guard(#{}).        1       100  12255 Ki    0.24%  12251 Ki 12320 Ki       8 ns   100%
test:noguard(#{}).      1       100  12204 Ki    0.36%  12215 Ki 12238 Ki       8 ns   100%
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7

Yes, that is where the SSA code for matching a map is generated. The first instruction for matching any map is always the is_map test.

Another line that may be interesting is the start of the pattern matching compilation.

In this context, Map = ~{}~ means matching of a map, not constructing an empty map. As I mentioned earlier, the matching of any map starts with the is_map test, so this is not an optimization.

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8

Thanks for all the replies and links. It is very helpful. beam_core_to_ssa.erl looks like quite complex. Lucky we don’t need to master this to write efficient Erlang code. Thanks Erlang/OTP team for doing all these heavy lifting implementations!

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9

You can also use this online tool to explore the result of various compilation stages. There you may see that both functions start to look equivalent in the SSA stage

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10

Or

$ erlc -S test.erl
$ cat test.S
...
{function, noguard, 1, 2}.
  {label,1}.
    {line,[{location,"test.erl",5}]}.
    {func_info,{atom,test},{atom,noguard},1}.
  {label,2}.
    {test,is_map,{f,1},[{x,0}]}.
    {move,{atom,ok},{x,0}}.
    return.


{function, guard, 1, 4}.
  {label,3}.
    {line,[{location,"test.erl",7}]}.
    {func_info,{atom,test},{atom,guard},1}.
  {label,4}.
    {test,is_map,{f,3},[{x,0}]}.
    {move,{atom,ok},{x,0}}.
    return.
...
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