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[MC] Resolve the difference of symbols in consecutive MCDataFragements

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Try to resolve the difference of two symbols in consecutive MCDataFragments.
This is important for an idiom like "foo:instr; .if . - foo; instr; .endif"
(https://bugs.llvm.org/show_bug.cgi?id=43795).

Reviewed By: nickdesaulniers

Differential Revision: https://reviews.llvm.org/D69411
This commit is contained in:
Jian Cai 2020-09-09 11:58:22 -07:00
parent cc25215e10
commit cec86ef133
5 changed files with 108 additions and 35 deletions
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7
include/llvm/MC/MCFragment.h
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@ -64,6 +64,10 @@ private:
/// The layout order of this fragment.
unsigned LayoutOrder;
/// The subsection this fragment belongs to. This is 0 if the fragment is not
// in any subsection.
unsigned SubsectionNumber = 0;
FragmentType Kind;
/// Whether fragment is being laid out.
@ -102,6 +106,9 @@ public:
bool hasInstructions() const { return HasInstructions; }
void dump() const;
void setSubsectionNumber(unsigned Value) { SubsectionNumber = Value; }
unsigned getSubsectionNumber() const { return SubsectionNumber; }
};
class MCDummyFragment : public MCFragment {

79
lib/MC/MCExpr.cpp
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@ -588,12 +588,7 @@ static void AttemptToFoldSymbolOffsetDifference(
if (!Asm->getWriter().isSymbolRefDifferenceFullyResolved(*Asm, A, B, InSet))
return;
MCFragment *FA = SA.getFragment();
MCFragment *FB = SB.getFragment();
if (FA == FB && !SA.isVariable() && !SA.isUnset() && !SB.isVariable() &&
!SB.isUnset()) {
Addend += (SA.getOffset() - SB.getOffset());
auto FinalizeFolding = [&]() {
// Pointers to Thumb symbols need to have their low-bit set to allow
// for interworking.
if (Asm->isThumbFunc(&SA))
@ -607,11 +602,17 @@ static void AttemptToFoldSymbolOffsetDifference(
// Clear the symbol expr pointers to indicate we have folded these
// operands.
A = B = nullptr;
return;
}
};
if (!Layout)
return;
const MCFragment *FA = SA.getFragment();
const MCFragment *FB = SB.getFragment();
// If both symbols are in the same fragment, return the difference of their
// offsets
if (FA == FB && !SA.isVariable() && !SA.isUnset() && !SB.isVariable() &&
!SB.isUnset()) {
Addend += SA.getOffset() - SB.getOffset();
return FinalizeFolding();
}
const MCSection &SecA = *FA->getParent();
const MCSection &SecB = *FB->getParent();
@ -619,30 +620,46 @@ static void AttemptToFoldSymbolOffsetDifference(
if ((&SecA != &SecB) && !Addrs)
return;
// One of the symbol involved is part of a fragment being laid out. Quit now
// to avoid a self loop.
if (!Layout->canGetFragmentOffset(FA) || !Layout->canGetFragmentOffset(FB))
return;
if (Layout) {
// One of the symbol involved is part of a fragment being laid out. Quit now
// to avoid a self loop.
if (!Layout->canGetFragmentOffset(FA) || !Layout->canGetFragmentOffset(FB))
return;
// Eagerly evaluate.
Addend += Layout->getSymbolOffset(A->getSymbol()) -
Layout->getSymbolOffset(B->getSymbol());
if (Addrs && (&SecA != &SecB))
Addend += (Addrs->lookup(&SecA) - Addrs->lookup(&SecB));
// Eagerly evaluate when layout is finalized.
Addend += Layout->getSymbolOffset(A->getSymbol()) -
Layout->getSymbolOffset(B->getSymbol());
if (Addrs && (&SecA != &SecB))
Addend += (Addrs->lookup(&SecA) - Addrs->lookup(&SecB));
// Pointers to Thumb symbols need to have their low-bit set to allow
// for interworking.
if (Asm->isThumbFunc(&SA))
Addend |= 1;
FinalizeFolding();
} else {
// When layout is not finalized, our ability to resolve differences between
// symbols is limited to specific cases where the fragments between two
// symbols (including the fragments the symbols are defined in) are
// fixed-size fragments so the difference can be calculated. For example,
// this is important when the Subtarget is changed and a new MCDataFragment
// is created in the case of foo: instr; .arch_extension ext; instr .if . -
// foo.
if (SA.isVariable() || SA.isUnset() || SB.isVariable() || SB.isUnset() ||
FA->getKind() != MCFragment::FT_Data ||
FB->getKind() != MCFragment::FT_Data ||
FA->getSubsectionNumber() != FB->getSubsectionNumber())
return;
// Try to find a constant displacement from FA to FB, add the displacement
// between the offset in FA of SA and the offset in FB of SB.
int64_t Displacement = SA.getOffset() - SB.getOffset();
for (auto FI = FB->getIterator(), FE = SecA.end(); FI != FE; ++FI) {
if (&*FI == FA) {
Addend += Displacement;
return FinalizeFolding();
}
// If symbol is labeled as micromips, we set low-bit to ensure
// correct offset in .gcc_except_table
if (Asm->getBackend().isMicroMips(&SA))
Addend |= 1;
// Clear the symbol expr pointers to indicate we have folded these
// operands.
A = B = nullptr;
if (FI->getKind() != MCFragment::FT_Data)
return;
Displacement += cast<MCDataFragment>(FI)->getContents().size();
}
}
}
static bool canFold(const MCAssembler *Asm, const MCSymbolRefExpr *A,

1
lib/MC/MCSection.cpp
View File

@ -82,6 +82,7 @@ MCSection::getSubsectionInsertionPoint(unsigned Subsection) {
SubsectionFragmentMap.insert(MI, std::make_pair(Subsection, F));
getFragmentList().insert(IP, F);
F->setParent(this);
F->setSubsectionNumber(Subsection);
}
return IP;

52
test/MC/ARM/directive-if-subtraction.s Normal file
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@ -0,0 +1,52 @@
// RUN: llvm-mc -triple armv7a-linux-gnueabihf %s -filetype=obj -o /dev/null 2>&1 | FileCheck --check-prefix=OBJ --allow-empty %s
// RUN: not llvm-mc -triple armv7a-linux-gnueabihf %s -o /dev/null 2>&1 | FileCheck --check-prefix=ASM %s
// RUN: llvm-mc -triple armv7a-linux-gnueabihf %s -filetype=obj -o - | llvm-objdump -d - | FileCheck --check-prefix=DISASM %s
nop
// Create a new MCDataFragment due to Subtarget change
.arch_extension sec
9997:nop
.if . - 9997b == 0
// OBJ-NOT:[[@LINE-1]]:5: error: expected absolute expression
// ASM:[[@LINE-2]]:5: error: expected absolute expression
// DISASM: orr r1, r1, #2
orr r1, r1, #1
.else
orr r1, r1, #2
.endif
@ RUN: not llvm-mc -filetype=obj -triple arm-linux-gnueabihf --defsym=ERR=1 %s -o /dev/null 2>&1 | FileCheck --check-prefix=ARM-ERR %s
@ RUN: not llvm-mc -filetype=obj -triple thumbv7a-linux-gnueabihf --defsym=ERR=1 %s -o /dev/null 2>&1 | FileCheck --check-prefix=THUMB2-ERR %s
.ifdef ERR
9997: nop
.align 4
nop
.if . - 9997b == 4
// ARM-ERR:[[@LINE-1]]:5: error: expected absolute expression
.endif
9997: nop
.space 4
nop
.if . - 9997b == 4
// ARM-ERR:[[@LINE-1]]:5: error: expected absolute expression
.endif
9997:
ldr r0,=0x12345678
.ltorg
nop
.if . - 9997b == 4
// ARM-ERR:[[@LINE-1]]:5: error: expected absolute expression
.endif
9997: nop
b external
nop
.if . - 9997b == 4
// THUMB2-ERR:[[@LINE-1]]:5: error: expected absolute expression
.endif
.endif

4
test/MC/MachO/reloc-diff.s
View File

@ -22,9 +22,5 @@ Ltemp:
// CHECK-NEXT: 0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
// CHECK-NEXT: 0x8 0 2 n/a GENERIC_RELOC_LOCAL_SECTDIFF 1 0x0
// CHECK-NEXT: 0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
// CHECK-NEXT: 0x4 0 2 n/a GENERIC_RELOC_LOCAL_SECTDIFF 1 0x0
// CHECK-NEXT: 0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
// CHECK-NEXT: 0x0 0 2 n/a GENERIC_RELOC_SECTDIFF 1 0x0
// CHECK-NEXT: 0x0 0 2 n/a GENERIC_RELOC_PAIR 1 0x0
// CHECK-NEXT: }
// CHECK-NEXT: ]