[llvm-commits] [llvm] r123294 - in /llvm/trunk: lib/MC/ELFObjectWriter.cpp lib/Target/ARM/ARMAsmBackend.cpp lib/Target/ARM/ARMAsmPrinter.cpp lib/Target/ARM/ARMFixupKinds.h lib/Target/ARM/ARMMCCodeEmitter.cpp test/MC/ARM/elf-movt.s

[Jason Kim]

On Wed, Jan 12, 2011 at 3:56 PM, Evan Cheng <evan.cheng at apple.com> wrote:
>
> On Jan 12, 2011, at 2:34 PM, Jason Kim wrote:
>
>>>
>>
>> The long story is quite convoluted, but the simple answer is that when
>> the "immediate value" being loaded to a register via movw/movt is
>> itself an expression that evaluates to a PCrelative value,
>> GNU as creates two different relocation values for the MOVW/MOVT. This
>> patch is to address that case.
>> It just so happens that some 99.999% of such addresses, the
>> expressions are of the form:
>>
>> movw r0, :lower16:Foo-(Bar+8)
>> movt  r0, :upper16:Foo-(Bar+8)
>
> Is this what gcc generates? That can't be right.
>
> movw r0, :lower16:Foo-(Bar+8)
> shouldn't be mean
> movw r0, :lower16:(Foo-(Bar+8))

Yes canonically speaking the latter is the least confusing of the two
- GNU as accepts both AFAIK.
-Jason

>
> Evan
>
>>
>> For such expressions, GNU as creates two different relocations with
>> the reloc tags
>> R_ARM_MOVW_PREL_NC and  R_ARM_MOVT_PREL,
>> In normal cases where the expression is not a pcrel value, GNU as
>> creates R_ARM_MOVT_ABS and R_ARM_MOVW_ABS_NC
>>
>> The reason why that hack routine works is because in nearly all cases,
>> the expression winds up being a highly constrained Binary expression -
>> LLVM also constrains this because MCValue has room for exactly two
>> symbols (but in reality, the pcrel expression can be hairy beasts like
>> Foo+Bar-Baz + 8 etc... but llvm doesn't seem to support this directly)
>>
>>>
>>>> +static bool EvaluateAsPCRel(const MCExpr *Expr) {
>>>> +  switch (Expr->getKind()) {
>>>> +  case MCExpr::SymbolRef: return false;
>>>> +  case MCExpr::Binary: return true;
>>>> +  default: assert(0 && "Unexpected expression type");
>>>> +  }
>>>> +}
>>>> +
>>>> uint32_t ARMMCCodeEmitter::
>>>> getMovtImmOpValue(const MCInst &MI, unsigned OpIdx,
>>>>                   SmallVectorImpl<MCFixup> &Fixups) const {
>>>> @@ -635,18 +661,27 @@
>>>>   if (MO.isImm()) {
>>>>     return static_cast<unsigned>(MO.getImm());
>>>>   } else if (const MCSymbolRefExpr *Expr =
>>>> -             dyn_cast<MCSymbolRefExpr>(MO.getExpr())) {
>>>> +             FindLHSymExpr(MO.getExpr())) {
>>>> +    // FIXME: :lower16: and :upper16: should be applicable to
>>>> +    // to whole expression, not just symbolrefs
>>>> +    // Until that change takes place, this hack is required to
>>>> +    // generate working code.
>>>> +    const MCExpr *OrigExpr = MO.getExpr();
>>>>     MCFixupKind Kind;
>>>>     switch (Expr->getKind()) {
>>>>     default: assert(0 && "Unsupported ARMFixup");
>>>>     case MCSymbolRefExpr::VK_ARM_HI16:
>>>>       Kind = MCFixupKind(ARM::fixup_arm_movt_hi16);
>>>> +      if (EvaluateAsPCRel(OrigExpr))
>>>> +        Kind = MCFixupKind(ARM::fixup_arm_movt_hi16_pcrel);
>>>>       break;
>>>>     case MCSymbolRefExpr::VK_ARM_LO16:
>>>>       Kind = MCFixupKind(ARM::fixup_arm_movw_lo16);
>>>> +      if (EvaluateAsPCRel(OrigExpr))
>>>> +        Kind = MCFixupKind(ARM::fixup_arm_movw_lo16_pcrel);
>>>>       break;
>>>>     }
>>>> -    Fixups.push_back(MCFixup::Create(0, Expr, Kind));
>>>> +    Fixups.push_back(MCFixup::Create(0, OrigExpr, Kind));
>>>>     return 0;
>>>>   };
>>>>   llvm_unreachable("Unsupported MCExpr type in MCOperand!");
>>>>
>>>> Modified: llvm/trunk/test/MC/ARM/elf-movt.s
>>>> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/MC/ARM/elf-movt.s?rev=123294&r1=123293&r2=123294&view=diff
>>>> ==============================================================================
>>>> --- llvm/trunk/test/MC/ARM/elf-movt.s (original)
>>>> +++ llvm/trunk/test/MC/ARM/elf-movt.s Tue Jan 11 18:19:25 2011
>>>> @@ -1,4 +1,6 @@
>>>> @ RUN: llvm-mc %s -triple=armv7-linux-gnueabi | FileCheck -check-prefix=ASM %s
>>>> +@ RUN: llvm-mc %s -triple=armv7-linux-gnueabi -filetype=obj -o - | \
>>>> +@ RUN:    elf-dump --dump-section-data | FileCheck -check-prefix=OBJ %s
>>>>       .syntax unified
>>>>       .text
>>>>       .globl  barf
>>>> @@ -12,3 +14,26 @@
>>>> @ ASM:          movw    r0, :lower16:GOT-(.LPC0_2+8)
>>>> @ ASM-NEXT:     movt    r0, :upper16:GOT-(.LPC0_2+16)
>>>>
>>>> +@@ make sure that the text section fixups are sane too
>>>> +@ OBJ:                 '.text'
>>>> +@ OBJ-NEXT:            'sh_type', 0x00000001
>>>> +@ OBJ-NEXT:            'sh_flags', 0x00000006
>>>> +@ OBJ-NEXT:            'sh_addr', 0x00000000
>>>> +@ OBJ-NEXT:            'sh_offset', 0x00000034
>>>> +@ OBJ-NEXT:            'sh_size', 0x00000008
>>>> +@ OBJ-NEXT:            'sh_link', 0x00000000
>>>> +@ OBJ-NEXT:            'sh_info', 0x00000000
>>>> +@ OBJ-NEXT:            'sh_addralign', 0x00000004
>>>> +@ OBJ-NEXT:            'sh_entsize', 0x00000000
>>>> +@ OBJ-NEXT:            '_section_data', 'f00f0fe3 ec0f4fe3'
>>>> +
>>>> +@ OBJ:              Relocation 0x00000000
>>>> +@ OBJ-NEXT:         'r_offset', 0x00000000
>>>> +@ OBJ-NEXT:         'r_sym'
>>>> +@ OBJ-NEXT:         'r_type', 0x0000002d
>>>> +
>>>> +@ OBJ:              Relocation 0x00000001
>>>> +@ OBJ-NEXT:         'r_offset', 0x00000004
>>>> +@ OBJ-NEXT:         'r_sym'
>>>> +@ OBJ-NEXT:         'r_type', 0x0000002e
>>>> +
>>>>
>>>>
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>>>
>>>
>>
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