Re^3: Freeing memory, revisited (Linux)

Nice theory

Simply the truth.

In your run-time example, a different mechanism causes the allocations to be retained. Every sub-expression that returns a result has a "pad temp" allocated to it to store the temporary result. The values in the pad temp are retained until the sub is freed, but they are re-used. i.e. in the following:

sub f { return "x" x $_[0] }
f(100_000); f(5); f(200_000);
[download]

a single temp will be stored within f's pad. On the first call it will hold a scalar of length 100_000, which gets retained. On the second call, the scalar is shrunk to length 5 (but not freed), and on the third call the scalar is realloced to grow to 200_000, and finally freed when the sub f is destroyed.

Dave.

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Re^4: Freeing memory, revisited (Linux) by Eliya (Vicar) on May 05, 2011 at 15:47 UTC
Simply the truth. So if constant folding (+ allocation) and storing the results as part of the code happens at compile time, how would you explain that the (runtime-)prints in the strace output of the following snippet are interleaved with the allocations? `{print STDERR "1"; "x" x 220} {print STDERR "2"; "x" x 220} {print STDERR "3"; "x" x 2**20}` [download] $ strace -e mmap,munmap,write ./903114.pl ... write(2, "1", 11) = 1 mmap(NULL, 1052672, PROT_READ\|PROT_WRITE, MAP_PRIVATE\|MAP_ANONYMOUS, - +1, 0) = 0x7fe6d7315000 write(2, "2", 12) = 1 mmap(NULL, 1052672, PROT_READ\|PROT_WRITE, MAP_PRIVATE\|MAP_ANONYMOUS, - +1, 0) = 0x7fe6d5e0f000 write(2, "3", 13) = 1 mmap(NULL, 1052672, PROT_READ\|PROT_WRITE, MAP_PRIVATE\|MAP_ANONYMOUS, - +1, 0) = 0x7fe6d5d0e000 munmap(0x7fe6d5d0e000, 1052672) = 0 munmap(0x7fe6d5e0f000, 1052672) = 0 munmap(0x7fe6d7315000, 1052672) = 0 [download] Also, `$ perl -MO=Concise,-exec ./903114.pl ... 11 <$> const[PV "x"] s 12 <$> const[IV 1048576] s 13 <2> repeat[t9] vK/2 <--- ...` [download] seems to suggest that the repetition operator is applied at runtime.	[reply] [d/l] [select]
Re^5: Freeing memory, revisited (Linux) by dave_the_m (Monsignor) on May 05, 2011 at 16:44 UTC
Ah, so repeat isn't constant folded. In which case, it's always the pad temps that retain the allocations. Dave.	[reply]