Incompatible behavior and warning for '#pragma GCC diagnostic pop' with no corresponding push

[llvmbot]

Bugzilla Link	22691
Version	3.4
OS	Linux
Reporter	LLVM Bugzilla Contributor
CC	@DougGregor

Extended Description

https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html states that "if a pop has no matching push, the command-line options are restored". This matches the actual behavior for GCC (provided you extend it to also mean that defaults are restored for things not specified on the command-line, which makes sense).

In clang, a pop without a corresponding push generates a warning and does not seem to restore warnings.

Test case:

void f(void) {
#pragma GCC diagnostic ignored "-Wunused-value"
    0;
#pragma GCC diagnostic pop
    0;
}

GCC generates a warning for the second '0;'. clang only generates a "warning: pragma diagnostic pop could not pop, no matching push".

[llvmbot]

clang has the warning in -Wunknown-pragmas. Perhaps it would make more sense in -Wpragrams if you want to keep it (documented as -Wno-pragmas in GCC: "Do not warn about misuses of pragmas, such as incorrect parameters, invalid syntax, or conflicts between pragmas. See also -Wunknown-pragmas"). clang doesn't recognize -W(no-)pragmas though.

With or without the warning, I think it should match the GCC behavior otherwise.

[llvmbot]

@llvm/issue-subscribers-clang-frontend

Author: None (llvmbot)

| | | | --- | --- | | Bugzilla Link | [22691](https://llvm.org/bz22691) | | Version | 3.4 | | OS | Linux | | Reporter | LLVM Bugzilla Contributor | | CC | @DougGregor |

Extended Description

https://gcc.gnu.org/onlinedocs/gcc/Diagnostic-Pragmas.html states that "if a pop has no matching push, the command-line options are restored". This matches the actual behavior for GCC (provided you extend it to also mean that defaults are restored for things not specified on the command-line, which makes sense).

In clang, a pop without a corresponding push generates a warning and does not seem to restore warnings.

Test case:

void f(void) {
#pragma GCC diagnostic ignored "-Wunused-value"
    0;
#pragma GCC diagnostic pop
    0;
}

GCC generates a warning for the second '0;'. clang only generates a "warning: pragma diagnostic pop could not pop, no matching push".