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bpo-29939: suppress compiler warnings in _ctypes_test#902

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benjamin-ctypes-warnings
Mar 31, 2017
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bpo-29939: suppress compiler warnings in _ctypes_test#902
vsajip merged 1 commit intomasterfrom
benjamin-ctypes-warnings

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@benjaminp benjaminp commented Mar 30, 2017

e.g.: _ctypes_test.c:53:42: warning: parameter ‘in’ set but not used [-Wunused-but-set-parameter]

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suppress compiler warnings in _ctypes_test
acfcdde 
e.g.: _ctypes_test.c:53:42: warning: parameter ‘in’ set but not used [-Wunused-but-set-parameter]
@serhiy-storchaka serhiy-storchaka requested a review from vsajip March 30, 2017 06:32
@serhiy-storchaka serhiy-storchaka changed the title suppress compiler warnings in _ctypes_test bpo-29939: suppress compiler warnings in _ctypes_test Mar 30, 2017
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On my system, at least, you can avoid the warning with just the simple:

    (void) in; /* avoid compiler warning */
    in.first = 0x0badf00d;
    in.second = 0x0badf00d;
    in.third = 0x0badf00d;

in the body of the function. Any reason that isn't preferable?

@brettcannon brettcannon added the type-feature A feature request or enhancement label Mar 30, 2017
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benjaminp commented Mar 31, 2017

The main advantage is that it forces the compiler to generate code to write the struct argument, which I gather is part of the reason for the test. If you don't care, you might as well remove the assignments in the body of the function—you can bet the compiler will with the current code—and avoid the warnings in the first place.

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vsajip commented Mar 31, 2017 •
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Interesting. Before I put in the fix which this test exercises, I added this test, and it failed - indicating that the compiler was generating the code to write to the struct argument. After I put in the fix, the test started passing. Perhaps I've missed something there.

If you are finding that the compiler optimises out all the assignments, what compiler version/compiler flags/architecture are you using? It would be useful to know. My testing was done with Ubuntu 16.04 x86_64 with stock gcc.

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benjaminp commented Mar 31, 2017 

$ cat test.c
typedef struct {
    unsigned long first;
    unsigned long second;
    unsigned long third;
} Test;

int example(Test in) {
    in.first = 0x0badf00d;
    in.second = 0x0badf00d;
    in.third = 0x0badf00d;
}
$ gcc -O2 -S test.c
$ cat test.s
	.file	"test.c"
	.text
	.p2align 4,,15
	.globl	example
	.type	example, @function
example:
.LFB0:
	.cfi_startproc
	rep ret
	.cfi_endproc
.LFE0:
	.size	example, .-example
	.ident	"GCC: (Ubuntu 6.2.0-5ubuntu12) 6.2.0 20161005"
	.section	.note.GNU-stack,"",@progbits

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vsajip commented Mar 31, 2017

Can't argue with that :-) I'm probably mis-remembering what happened when I did the test.

@vsajip vsajip merged commit 164d30e into master Mar 31, 2017
@vsajip vsajip deleted the benjamin-ctypes-warnings branch March 31, 2017 11:05
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