[LLVMdev] MIPS n64 ABI and non-PIC

[Daniel Sanders]

Ok, I didn't know that. It looks like gcc's behaviour is triple-dependent. My mips-mti-linux-gnu-gcc toolchain (last updated in January) always generates PIC code for –fPIC, -fno-PIC, and neither option. I've just tried the latest mips-sde-elf-gcc toolchain and found it emits non-PIC code for –fno-PIC and neither option, and errors out with –fPIC (without additional options).

From: Brandon Hill [mailto:bgh.list.reader at gmail.com]
Sent: 02 May 2014 19:53
To: Daniel Sanders
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] MIPS n64 ABI and non-PIC

Actually, GCC will generate non-PIC for n64.  Maybe that is a recent addition, but we are using its results.  Even if PIC may be faster and smaller code, it seems that non-PIC is still useful for bare-metal.  That's the driver of my interest.  I guess we can just test what happens when that part of the conditional is removed.  As a side note, if it isn't supported then we should probably have the compiler warn the user that the two flags aren't compatible.
Brandon

On Wed, Apr 30, 2014 at 2:25 AM, Daniel Sanders <Daniel.Sanders at imgtec.com<mailto:Daniel.Sanders at imgtec.com>> wrote:
GCC does the same thing. I haven't found anything written down that explains this yet but I believe it's that PIC consistently generates faster and smaller code than non-PIC for N64.

For example, a non-PIC implementation of getAddrLocal() would probably generate something like this:
lui $1, %highest(foo)
add $1, $1, %higher(foo)
dsll $1, $1, 32
lui $2, %hi(foo)
add $2, $2, %lo(foo)
add $1, $1, $2
which is 6 instructions per-symbol referenced. The current PIC implementation generates this:
lui $1, %hi(%neg(%gp_rel(bar)))
daddu $1, $1, $25
daddiu $1, $1, %lo(%neg(%gp_rel(bar)))
ld $2, %got_disp(foo)($1)
which is a one-time cost of 3 instructions to set up the GOT pointer, plus one load per-symbol referenced.

From: llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu> [mailto:llvmdev-bounces at cs.uiuc.edu<mailto:llvmdev-bounces at cs.uiuc.edu>] On Behalf Of Brandon Hill
Sent: 29 April 2014 22:35
To: llvmdev at cs.uiuc.edu<mailto:llvmdev at cs.uiuc.edu>
Subject: [LLVMdev] MIPS n64 ABI and non-PIC

Has anyone experimented with generating non-PIC for MIPS64 and the n64 ABI? Currently MipsISelLowering.cpp uses conditions like:

 if ((getTargetMachine().getRelocationModel() == Reloc::PIC_) || IsN64) {
 }

around any PIC code generation.  Is generating non-PIC just untested, or is it known not to work?  I can't find any discussion of it anywhere. I ran into this when trying to see why --relocation-model=static had no effect on the output.  I'm game to test it, but it would help to know that it isn't pointless due to a significant known issue.
Brandon


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