[llvm-commits] [llvm] r123424 - in /llvm/trunk: include/llvm/Object/MachOFormat.h lib/MC/ELFObjectWriter.cpp lib/MC/MachObjectWriter.cpp lib/Target/ARM/ARMAsmBackend.cpp lib/Target/ARM/ARMFixupKinds.h lib/Target/ARM/ARMMCCodeEmitter.cpp test/MC/A

[Kevin Enderby]

On Jan 18, 2011, at 2:48 PM, Evan Cheng wrote:

> 
> On Jan 18, 2011, at 12:22 PM, Jason Kim wrote:
> 
>> On Fri, Jan 14, 2011 at 3:38 PM, Evan Cheng <evan.cheng at apple.com> wrote:
>>> 
>>> On Jan 14, 2011, at 12:20 PM, Jason Kim wrote:
>>> 
>>> 
>>> Hi Evan,
>>> 
>>> FYI, the movt hi16 fixup you put in does not match the output of GNU as -
>>> Let me look into this and figure out why. I'll reply soon.
>>> 
>>> -jason
>>> 
>>> 
>>> Interesting. That's probably due to this:
>>> 
>>>  case ARM::fixup_arm_movt_hi16:
>>> 
>>> -  case ARM::fixup_arm_movw_lo16:
>>> 
>>>  case ARM::fixup_arm_movt_hi16_pcrel:
>>> 
>>> +    Value >>= 16;
>>> 
>>> +    // Fallthrough
>>> 
>>> +  case ARM::fixup_arm_movw_lo16:
>>> 
>>>  case ARM::fixup_arm_movw_lo16_pcrel: {
>>> 
>>>    unsigned Hi4 = (Value & 0xF000) >> 12;
>>> 
>>>    unsigned Lo12 = Value & 0x0FFF;
>>> 
>>> @@ -168,6 +175,26 @@
>>> 
>>>    Value = (Hi4 << 16) | (Lo12);
>>> 
>>>    return Value;
>>> 
>>>  }
>>> 
>>> Notice the Value is shifted down 16 bits for movt. I'm fairly sure this is
>>> right, at least for Darwin. It's possible there is a bug elsewhere.
>>> Evan
>>> 
>>> 
>> 
>> Hmm, I see at least one problem that was not caught earlier -
>> 
>> 1) movw/movt paired with :lower16:/:upper16:
>> 2) movw/movt NOT paired with a :lower16:/:upper16:
>> 
>> The handling has to be different for these two cases
>> in case 2, the constant will already be a 16bit value
>> in case 1, the constant will need to be right shifted (during? before? fixup)
>> 
>> It seems like this needs to be handled before the fixup actually happens.
>> 
>> Does Darwin assembler require :lower16:/:upper16: when using movw/movt?
> 
> I believe so. Kevin?

The Darwin assembler does not "require" a lower16:/:upper16: when using movw/movt if that is used with just const:

% cat x.s
.syntax unified
.code 16
.thumb_func t
t:
	movt	r1,#1
	movw	r2,#2

% as -arch armv7 x.s

% otool -tv a.out
a.out:
(__TEXT,__text) section
t:
00000000	f2c00101	movt	r1, #1	; 0x1
00000004	f2400202	movw	r2, #2	; 0x2

But normally is used with a symbol or expression in any generated code for example

> 
> Evan
> 
>> 
>> -jason
>