code review 93200044: cmd/gc: disable link-time copying of un-Go-initialized ...

cmd/gc: disable link-time copying of un-Go-initialized globals

If you write:

        var x = 3

then the compiler arranges for x to be initialized in the linker
with an actual 3 from the data segment, rather than putting
x in the bss and emitting init-time "x = 3" assignment code.

If you write:

        var y = x
        var x = 3

then the compiler is clever and treats this the same as if
the code said 'y = 3': they both end up in the data segment
with no init-time assignments.

If you write

        var y = x
        var x int

then the compiler was treating this the same as if the
code said 'x = 0', making both x and y zero and avoiding
any init-time assignment.

This copying optimization to avoid init-time assignment of y
is incorrect if 'var x int' doesn't mean 'x = 0' but instead means
'x is initialized in C or assembly code'. The program ends up
with 'y = 0' instead of 'y = the value specified for x in that other code'.

Disable the propagation if there is no initializer for x.

This comes up in some uses of cgo, because cgo generates
Go globals that are initialized in accompanying C files.

Fixes issue 7665.

[rsc, 2014-05-09 19:33:13]

Hello golang-codereviews@googlegroups.com,

I'd like you to review this change to
https://code.google.com/p/go/

[iant, 2014-05-09 19:54:55]

LGTM

[rsc, 2014-05-09 20:03:45]

*** Submitted as https://code.google.com/p/go/source/detail?r=be19ef9bd7b3 ***

cmd/gc: disable link-time copying of un-Go-initialized globals

If you write:

        var x = 3

then the compiler arranges for x to be initialized in the linker
with an actual 3 from the data segment, rather than putting
x in the bss and emitting init-time "x = 3" assignment code.

If you write:

        var y = x
        var x = 3

then the compiler is clever and treats this the same as if
the code said 'y = 3': they both end up in the data segment
with no init-time assignments.

If you write

        var y = x
        var x int

then the compiler was treating this the same as if the
code said 'x = 0', making both x and y zero and avoiding
any init-time assignment.

This copying optimization to avoid init-time assignment of y
is incorrect if 'var x int' doesn't mean 'x = 0' but instead means
'x is initialized in C or assembly code'. The program ends up
with 'y = 0' instead of 'y = the value specified for x in that other code'.

Disable the propagation if there is no initializer for x.

This comes up in some uses of cgo, because cgo generates
Go globals that are initialized in accompanying C files.

Fixes issue 7665.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/93200044

[gobot, 2014-05-09 20:43:50]

This CL appears to have broken the openbsd-amd64-rootbsd builder.
See http://build.golang.org/log/d1a49acdacfd61b40f18ee142daee36c15b21c4a