1242060 - Use #pragma once header guards in SpiderMonkey

Status: RESOLVED WONTFIX

(Core :: JavaScript Engine, defect)

Description

[Terrence Cole [:terrence]]

Attachment: pragma_once-v0.diff

The internet says that #pragma once has been safe to use basically forever in every compiler that I've even heard of. So let's try it out. Stuff builds locally, let's see what try has to say:
https://treeherder.mozilla.org/#/jobs?repo=try&revision=986d05d5eb77

You may not want to review every single line; turns out we have lots of headers. Like, way more headers than I would have thought possible:
 436 files changed, 437 insertions(+), 1746 deletions(-)

The script I used to make the change is:
'''bash
#!/bin/bash
FILES=`find . -name "*.h" | grep -v irreg | grep -v libffi | grep -v vixl | grep -v editline | grep -v vtune | grep -v style | grep -v llvm-compiler-rt | grep -v js-conf`

for FILE in $FILES; do
    GUARDNAME=`sed -n "s/#ifndef \(.*_h\)$/\1/p" ${FILE}`
    if [ -z $GUARDNAME ]; then
        exit 1
    fi

    grep -q "#endif....${GUARDNAME}" ${FILE} || exit 2

    sed -i "s/#ifndef ${GUARDNAME}$/#pragma once/" ${FILE}
    sed -i "/#define ${GUARDNAME}/d" ${FILE}
    sed -i "/#endif....${GUARDNAME}/d" ${FILE}
    sed -i -e :a -e '/^\n*$/{$d;N;ba' -e '}' ${FILE}
done
'''

One edit was required in CommonPropertyNames.h: g++ did not like EOF terminating the macro after I stripped the trailing whitespace.

Comment 1

[Andrew McCreight [:mccr8]]

There was a proposal to do this for Gecko back in 2012:
  https://groups.google.com/forum/#!topic/mozilla.dev.platform/PgDjWw3xp8k

I'm not sure if the objections there still apply or not, but it is at least worth reading over.

Comment 2

[Mike Hommey [:glandium]]

This comment still applies.
https://groups.google.com/d/msg/mozilla.dev.platform/PgDjWw3xp8k/PLYQc5xoWmsJ

Comment 3

[Terrence Cole [:terrence]]

https://treeherder.mozilla.org/#/jobs?repo=try&revision=1f5e5fafc3a9

Comment 4

[Terrence Cole [:terrence]]

(In reply to Mike Hommey [:glandium] from comment #2)
> This comment still applies.
> https://groups.google.com/d/msg/mozilla.dev.platform/PgDjWw3xp8k/PLYQc5xoWmsJ

Well, drat!