[Bug 970] #pragma visibility hidden in MdePkg/Include/X64/ProcessorBind.h is misconceived #pragma


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https://bugzilla.tianocore.org/show_bug.cgi?id=970

--- Comment #7 from zenith432(a)users.sourceforge.net ---
As you may be aware, the Tianocore crowd are obsessed with binary size, and
so even if GenFw deals with the relocations correctly, you should still
expect some pushback to removing this #pragma if it results in a code
size increase for non-LTO builds.
- I've examined the generated code - using -fpie eliminates nearly 100% of
GOTPCREL emitted by GCC, and it's a coincidence that some are left behind - and
that the ones left behind are not transformable by ld.

- It is also a coincidence that -fvisiblity=hidden eliminates the remaining
cases. There are code constructs not used in EDK2 that still emit GOTPCREL and
you haven't run into. I mentioned this in my first post. Take this example.

====== a.c
#define SIZE_OF_THUNK 9

extern void thunkArea(void);
extern long const offsetOfThunkTable;

unsigned long compute_thunk_address(long thunkIndexInTable)
{
return (unsigned long) thunkArea + offsetOfThunkTable * SIZE_OF_THUNK +
thunkIndexInTable * SIZE_OF_THUNK;
}
======
gcc -Os -fpie -fvisibility=hidden -c -o a.o a.c
objdump -dr a.o

0000000000000000 <compute_thunk_address>:
0: 48 6b 05 00 00 00 00 imul $0x9,0x0(%rip),%rax # 8
<compute_thunk_address+0x8>
7: 09
3: R_X86_64_PC32 offsetOfThunkTable-0x5
8: 48 8d 3c ff lea (%rdi,%rdi,8),%rdi
c: 48 03 05 00 00 00 00 add 0x0(%rip),%rax # 13
<compute_thunk_address+0x13>
f: R_X86_64_REX_GOTPCRELX thunkArea-0x4
13: 48 01 f8 add %rdi,%rax
16: c3 retq

With -fpie -fvisibility=hidden emitted GOTPCREL relocation with add instruction
that can't be eliminated by ld. GCC 8.1.1 as before.

I made the example a little more elaborate than needs to be, to make GCC emit
an "add" instruction. A simpler pointer computation sometimes emits a "mov"
instruction (which can be transformed by ld), and some other forms of pointer
arithmetic drop the GOTPCREL altogether.

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