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getauxv bloat in static exe #21522

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michaelortmann opened this issue Sep 26, 2024 · 2 comments · May be fixed by #22276
Open

getauxv bloat in static exe #21522

michaelortmann opened this issue Sep 26, 2024 · 2 comments · May be fixed by #22276
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bug Observed behavior contradicts documented or intended behavior os-linux standard library This issue involves writing Zig code for the standard library.

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@michaelortmann
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Zig Version

zig-0.14.0-dev.1646+b19d0fb0f

Steps to Reproduce and Observed Behavior

The following bugreport got lost in a comment of/after merged PR #16977

#16977 (comment)

This bug also contributes to #20711

Expected Behavior

If getauxval() code is not used and unreachable in the final exe, it should not end up there.
@michaelortmann michaelortmann added the bug Observed behavior contradicts documented or intended behavior label Sep 26, 2024
@rohlem
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rohlem commented Sep 26, 2024 •
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Wanted to mention that #20517 seems to have restricted it
from any build to only executables (and modules with root exposing main).

Is the suggestion to further restrict it to only @export it when building non-static (i.e. dynamic) artifacts?

I also think we could try moving the relevant comptime block into the container namespace used to define getauxval.
As far as I understand, that should make it analyzed exactly when the definition of getauxval that relies on it is analyzed.
(I think this would make some of the surrounding if condition unnecessary as well.)

@michaelortmann
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While i was not able to push it into the container definition, tt looks like the definition of getauxval in

zig/lib/std/os/linux.zig

Line 493 in 085cc54

}.getauxval else getauxvalImpl;
depends on the export before

zig/lib/std/os/linux.zig

Line 487 in 085cc54

@export(&getauxvalImpl, .{ .name = "getauxval", .linkage = .weak });
, which makes it complex.

But if we add the builtin.link_mode == .dynamic to

zig/lib/std/os/linux.zig

Line 486 in 085cc54

if (extern_getauxval and (builtin.output_mode == .Exe or @hasDecl(root, "main"))) {
and reverse the logic in

zig/lib/std/os/linux.zig

Line 491 in 085cc54

pub const getauxval = if (extern_getauxval) struct {
to if (!extern_getauxval) it could fix all cases.

At least it fixes the case of a static executable (and my little hello world test executable, when stripped, went down from 2280 to 2176 bytes). I commited a patch but didnt create a PR yet due to limited test / fear of regression elsewhere:

michaelortmann@59d0c85

@alexrp alexrp added os-linux standard library This issue involves writing Zig code for the standard library. labels Oct 9, 2024
@wooster0 wooster0 linked a pull request Dec 20, 2024 that will close this issue
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bug Observed behavior contradicts documented or intended behavior os-linux standard library This issue involves writing Zig code for the standard library.
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linux: omit getauxval from binary if not needed wooster0/zig
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@alexrp @rohlem @michaelortmann