[bug #64746] Exponential Runtime in make 4.4.1 when export is used

[Till Backhaus]

URL:
  <https://savannah.gnu.org/bugs/?64746>

                 Summary: Exponential Runtime in make 4.4.1 when export is
used
                   Group: make
               Submitter: tback
               Submitted: Wed 04 Oct 2023 09:31:42 PM UTC
                Severity: 3 - Normal
              Item Group: Bug
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any
       Component Version: 4.4.1
        Operating System: POSIX-Based
           Fixed Release: None
           Triage Status: None


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Follow-up Comments:


-------------------------------------------------------
Date: Wed 04 Oct 2023 09:31:42 PM UTC By: Till Backhaus <tback>
This is my first report here, I might lack the proper etiquette.

I'm on fedora 38 which installs gnu-make 4.4.1. I'm reporting a regression
related to the changed
behavior of export:

> * WARNING: Backward-incompatibility!
>  Previously makefile variables marked as export were not exported to
commands
>  started by the $(shell ...) function.  Now, all exported variables are
>  exported to $(shell ...).  If this leads to recursion during expansion,
then
>  for backward-compatibility the value from the original environment is
used.
>  To detect this change search for 'shell-export' in the .FEATURES variable.


This rather short Makefile shows exponential runtime depending on the number
of variables.


export

VAR_1 ?= $(shell echo 1)
VAR_2 ?= $(shell echo 2)
VAR_3 ?= $(shell echo 3)
VAR_4 ?= $(shell echo 4)
VAR_5 ?= $(shell echo 5)
VAR_6 ?= $(shell echo 6)
VAR_7 ?= $(shell echo 7)

foo:
        echo foo


Runtime with 7 variables is ~20s on reasonably current hardware (AMD Ryzen 7
4750U):


time make foo
echo foo
foo

real 0m21.663s
user 0m4.082s
sys 0m18.027s



make foo --debug=verbose
GNU Make 4.4.1
Built for x86_64-redhat-linux-gnu
Copyright (C) 1988-2023 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later
<https://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
Reading makefiles...
Reading makefile 'Makefile'...
Updating makefiles....
Updating goal targets....
Considering target file 'foo'.
 File 'foo' does not exist.
Finished prerequisites of target file 'foo'.
Must remake target 'foo'.
echo foo
Makefile:7: not recursively expanding VAR_5 to export to shell function
...
Makefile:4: not recursively expanding VAR_2 to export to shell function
foo
Successfully remade target file 'foo'.


The number of expansions grows exponentially:
I modified the Makefile to contain different numbers of variables and got
an exponential number of suppressed expansions:


make foo --debug=verbose | grep -c recurs
82201

Variables to Expansions:
1 var 1
2 vars 6
3 vars 33
4 vars 196
5 vars 1305
6 vars 9786
7 vars 82201


Best regards,
Till







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