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[LLD][ELF] Don't spill to same memory region #129795

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merged 2 commits into from
Mar 11, 2025
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[LLD][ELF] Don't spill to same memory region #129795

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f7e8896
[LLD][ELF] Don't spill to same memory region
mysterymath Mar 4, 2025
e833a6d
Test VMA vs LMA; add fix
mysterymath Mar 6, 2025
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34 changes: 25 additions & 9 deletions lld/ELF/LinkerScript.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -563,7 +563,7 @@ LinkerScript::computeInputSections(const InputSectionDescription *cmd,
continue;

if (!cmd->matchesFile(*sec->file) || pat.excludesFile(*sec->file) ||
sec->parent == &outCmd || !flagsMatch(sec))
!flagsMatch(sec))
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Just so I understand. These changes are not required for the fix, but they are no longer necessary because of the changes to spillSections()

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Yes, that's correct. This was code added to prevent this roughly this issue, but it appears to have been neither necessary nor sufficient.

As-is, this shouldn't create any behavioral differences, since spilling to a different memory region implies spilling to a different output section. But this also does hint towards a future way to express very detailed address constraints within an output section, which is something that has been on my radar for a while.

For example:

CLASS(text) ( *(.text) }
.text {
  CLASS(text)
  . = 0x1234; 
  *(.text.at_0x1234)
  CLASS(text)
}

The first pass would assign everything to the first CLASS reference, but this would cause dot to move backwards. The spiller could detect the amount dot moved backwards and spill that much from a preceding class, just like it does for memory region overages. The difference is that later references within the same output section may help to resolve this.

I don't think there's actually anything you could achieve with this that you couldn't achieve by splitting everything into fine-grained output sections, but to me it would just help to make spilling feel more orthogonal with linker features like dot assignment. Broadly, I'd tend to think that any address assignment failure should trigger spilling if that might have a chance to prevent the issue on the next pass.

continue;

if (sec->parent) {
Expand Down Expand Up @@ -626,7 +626,7 @@ LinkerScript::computeInputSections(const InputSectionDescription *cmd,

for (InputSectionDescription *isd : scd->sc.commands) {
for (InputSectionBase *sec : isd->sectionBases) {
if (sec->parent == &outCmd || !flagsMatch(sec))
if (!flagsMatch(sec))
continue;
bool isSpill = sec->parent && isa<OutputSection>(sec->parent);
if (!sec->parent || (isSpill && outCmd.name == "/DISCARD/")) {
Expand Down Expand Up @@ -1585,16 +1585,32 @@ bool LinkerScript::spillSections() {
if (isa<PotentialSpillSection>(isec))
continue;

// Find the next potential spill location and remove it from the list.
auto it = potentialSpillLists.find(isec);
if (it == potentialSpillLists.end())
continue;
break;

// Consume spills until finding one that might help, then consume it.
PotentialSpillList &list = it->second;
PotentialSpillSection *spill = list.head;
if (spill->next)
list.head = spill->next;
else
potentialSpillLists.erase(isec);
PotentialSpillSection *spill;
for (spill = list.head; spill; spill = spill->next) {
if (list.head->next)
list.head = spill->next;
else
potentialSpillLists.erase(isec);

// Spills to the same region that overflowed cannot help.
if (hasRegionOverflowed(osec->memRegion) &&
spill->getParent()->memRegion == osec->memRegion)
continue;
if (hasRegionOverflowed(osec->lmaRegion) &&
spill->getParent()->lmaRegion == osec->lmaRegion)
continue;

// This spill might resolve the overflow.
break;
}
if (!spill)
continue;

// Replace the next spill location with the spilled section and adjust
// its properties to match the new location. Note that the alignment of
Expand Down
100 changes: 100 additions & 0 deletions lld/test/ELF/linkerscript/section-class.test
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Original file line number Diff line number Diff line change
Expand Up @@ -450,3 +450,103 @@ SECTIONS {

# TO-DISCARD: error: section '.two_byte_section' cannot spill from/to /DISCARD/
# TO-DISCARD-WARN: warning: section '.two_byte_section' cannot spill from/to /DISCARD/

#--- same-mem-region.lds
## Spills to the same memory region that overflowed do not consume address assignment passes.
MEMORY {
a : ORIGIN = 0, LENGTH = 0
b : ORIGIN = 0, LENGTH = 3
c : ORIGIN = 3, LENGTH = 3
d : ORIGIN = 6, LENGTH = 3
}
SECTIONS {
CLASS(class) { *(.one_byte_section .two_byte_section) }
.a00 : { CLASS(class) } >a AT>c
.a01 : { CLASS(class) } >a AT>d
.a02 : { CLASS(class) } >a AT>d
.a03 : { CLASS(class) } >a AT>d
.a04 : { CLASS(class) } >a AT>d
.a05 : { CLASS(class) } >a AT>d
.a06 : { CLASS(class) } >a AT>d
.a07 : { CLASS(class) } >a AT>d
.a08 : { CLASS(class) } >a AT>d
.a09 : { CLASS(class) } >a AT>d
.a10 : { CLASS(class) } >a AT>d
.a11 : { CLASS(class) } >a AT>d
.a12 : { CLASS(class) } >a AT>d
.a13 : { CLASS(class) } >a AT>d
.a14 : { CLASS(class) } >a AT>d
.a15 : { CLASS(class) } >a AT>d
.a16 : { CLASS(class) } >a AT>d
.a17 : { CLASS(class) } >a AT>d
.a18 : { CLASS(class) } >a AT>d
.a19 : { CLASS(class) } >a AT>d
.a20 : { CLASS(class) } >a AT>d
.a21 : { CLASS(class) } >a AT>d
.a22 : { CLASS(class) } >a AT>d
.a23 : { CLASS(class) } >a AT>d
.a24 : { CLASS(class) } >a AT>d
.a25 : { CLASS(class) } >a AT>d
.a26 : { CLASS(class) } >a AT>d
.a27 : { CLASS(class) } >a AT>d
.a28 : { CLASS(class) } >a AT>d
.a29 : { CLASS(class) } >a AT>d
.a30 : { CLASS(class) } >a AT>d
.b : { CLASS(class) } >b AT>d
}

# RUN: ld.lld -T same-mem-region.lds -o same-mem-region spill.o
# RUN: llvm-readelf -S same-mem-region | FileCheck %s --check-prefix=SAME-MEM-REGION

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Is there a reasonable way to test the differences in mem region and lma region? For example an output section is >a AT> b I don't think it necessary has to be a within a pass limit though, just showing it is possible.

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Good idea, and this uncovered a subtlety. We should only skip entries with the same mem region if it is currently overflowing; same with lma region. Added a test and fix.

# SAME-MEM-REGION: Name Type Address Off Size
# SAME-MEM-REGION: .b PROGBITS 0000000000000000 001000 000003

#--- same-lma-region.lds
## Spills to the same load region that overflowed do not consume address assignment passes.
MEMORY {
a : ORIGIN = 0, LENGTH = 0
b : ORIGIN = 0, LENGTH = 3
c : ORIGIN = 3, LENGTH = 3
d : ORIGIN = 6, LENGTH = 3
}
SECTIONS {
CLASS(class) { *(.one_byte_section .two_byte_section) }
.a00 : { CLASS(class) } >c AT>a
.a01 : { CLASS(class) } >d AT>a
.a02 : { CLASS(class) } >d AT>a
.a03 : { CLASS(class) } >d AT>a
.a04 : { CLASS(class) } >d AT>a
.a05 : { CLASS(class) } >d AT>a
.a06 : { CLASS(class) } >d AT>a
.a07 : { CLASS(class) } >d AT>a
.a08 : { CLASS(class) } >d AT>a
.a09 : { CLASS(class) } >d AT>a
.a10 : { CLASS(class) } >d AT>a
.a11 : { CLASS(class) } >d AT>a
.a12 : { CLASS(class) } >d AT>a
.a13 : { CLASS(class) } >d AT>a
.a14 : { CLASS(class) } >d AT>a
.a15 : { CLASS(class) } >d AT>a
.a16 : { CLASS(class) } >d AT>a
.a17 : { CLASS(class) } >d AT>a
.a18 : { CLASS(class) } >d AT>a
.a19 : { CLASS(class) } >d AT>a
.a20 : { CLASS(class) } >d AT>a
.a21 : { CLASS(class) } >d AT>a
.a22 : { CLASS(class) } >d AT>a
.a23 : { CLASS(class) } >d AT>a
.a24 : { CLASS(class) } >d AT>a
.a25 : { CLASS(class) } >d AT>a
.a26 : { CLASS(class) } >d AT>a
.a27 : { CLASS(class) } >d AT>a
.a28 : { CLASS(class) } >d AT>a
.a29 : { CLASS(class) } >d AT>a
.a30 : { CLASS(class) } >d AT>a
.b : { CLASS(class) } >d AT>b
}

# RUN: ld.lld -T same-lma-region.lds -o same-lma-region spill.o
# RUN: llvm-readelf -S same-lma-region | FileCheck %s --check-prefix=SAME-LMA-REGION

# SAME-LMA-REGION: Name Type Address Off Size
# SAME-LMA-REGION: .b PROGBITS 0000000000000006 001006 000003