Macro expansion not performed on pragma arguments when using -save-temps

[llvmbot]

Bugzilla Link	19529
Version	3.7
OS	Linux
Attachments	C test case
Reporter	LLVM Bugzilla Contributor
CC	@rnk

Extended Description

Consider the following C code:

#define PACK_SZ 1
#pragma pack(PACK_SZ)
struct s1 {
char a;
int b;
};
struct s1 myst;

---

When compiling it without '-save-temps',
the pragma is correctly handled:

clang pragma_pack.c -O2 -S -emit-llvm
cat pragma_pack.ll
[...]
%struct.s1 = type <{ i8, i32 }>
@​myst = common global %struct.s1 zeroinitializer, align 1
[...]

---

When compiling it with '-save-temps', the macro
is not expanded and the pragma is then ignored:

clang pragma_pack.c -O2 -S -emit-llvm -save-temps
pragma_pack.c:4:14: warning: unknown action for '#pragma pack' - ignored
#pragma pack(PACK_SZ)
^
1 warning generated.
cat
[...]
%struct.s1 = type { i8, i32 }
@​myst = common global %struct.s1 zeroinitializer, align 4
[...]

The problem has been reproduced with other pragmas.
The problem is present in CLANG 3.4, but likely in other version as well.

[rnk]

See also #18950 , which I "fixed" by always expanding macros when -fms-extensions is set, because that's what MSVC does.

The problem is that, depending on the pragma, macro expansion may or may not occur. Only the parser knows. And, when we're preprocessing, the parser isn't around to tell us what to expand and what not to. We need to change the layering that the lexer knows how to treat each pragma.

[rnk]

Note that gcc does not expand macros here, which explains clang's current behavior:

$ cat t.cpp
#include <stdio.h>
#define PACK_SZ 1
#pragma pack(PACK_SZ)
struct s1 {
char a;
int b;
};
struct s1 myst;
int main() {
printf("%d\n", sizeof(myst));
}

$ g++ t.cpp -o t.exe
t.cpp:3:14: warning: unknown action ‘PACK_SZ’ for ‘#pragma pack’ - ignored [-Wpragmas]
#pragma pack(PACK_SZ)
^
$ ./t.exe
8