[LLVMdev] Typo in IsLegalToCallImmediateAddr?

David Meyer pdox at google.com
Mon Oct 17 16:24:54 PDT 2011


I believe your example is not related to IsLegalToCallImmediateAddr.

This is an example of calling to an immediate address:

typedef int (*funcptr)(void);

int main() {
  funcptr foo = (funcptr)0x100;

If IsLegalToCallImmedateAddr is true, this generates a call to
absolute address 0x100:

call 0x100

This requires a relocation of the value 0x100 - PC.
(NOTE: this is NOT the same as: "foo: call foo+0x100", which requires
no relocation)

This can't be done correctly in PIC mode. If you do this in a shared
library, the relocation is ignored and you get a jump to module_start
+ 0x100 instead of absolute 0x100.

- pdox

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