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[lld-macho]Define a flag for adjusting slop scale #164295

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[lld-macho]Define a flag for adjusting slop scale #164295

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d694297
[lld-macho]Define a macro to allow specifying the slop size at build …
oontvoo Oct 20, 2025
f3ddb55
make slop-scale a linker option instead of build-time flag + update …
oontvoo Oct 22, 2025
8bbafdd
Merge branch 'main' into thunk_size
oontvoo Oct 22, 2025
a9b939e
Update lld/MachO/Driver.cpp
oontvoo Oct 28, 2025
1436606
Update lld/MachO/Driver.cpp
oontvoo Oct 28, 2025
e5da104
Merge branch 'main' into thunk_size
oontvoo Oct 28, 2025
cb0f3fb
reformat
oontvoo Oct 28, 2025
0ddc940
reformat again
oontvoo Oct 28, 2025
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5 changes: 5 additions & 0 deletions lld/MachO/CMakeLists.txt
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Original file line number Diff line number Diff line change
@@ -1,3 +1,8 @@

option(LLD_MACHO_SLOP_FACTOR
"Specify the slop factor. For very large build, the default of 256 might be too small, which can cause thunk-range overrun. Recommend increasing this value to 1024 or higher as needed.", 256)
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@dmaclach dmaclach Oct 20, 2025

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Is there any better documentation we can provide as to what this actually is/does? For example do we know what is causing a thunk-range overrun? Is there some coding patterns that may cause it? This seems like we are just putting a band-aid on what may be a bad coding practice.

Also

"For very large builds" vs "For very large build".

is this something that should maybe be specified as a build flag vs a macro?

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@oontvoo oontvoo Oct 22, 2025

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Is there any better documentation we can provide as to what this actually is/does? For example do we know what is causing a thunk-range overrun? Is there some coding patterns that may cause it? This seems like we are just putting a band-aid on what may be a bad coding practice.

Ok, how about describing the root cause (ie., too many consec branches)?
As for coding patterns that might cause it, one of them could be not using subsection_via_symbols (or anything that prevent deadstripping), which would create a giant input-section, increasing your chance of hitting this thunk-range overrun error.

Updated the code + docs to reflect that.

add_definitions("-DLLD_MACHO_SLOP_FACTOR=${LLD_MACHO_SLOP_FACTOR}")

set(LLVM_TARGET_DEFINITIONS Options.td)
tablegen(LLVM Options.inc -gen-opt-parser-defs)
add_public_tablegen_target(MachOOptionsTableGen)
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7 changes: 6 additions & 1 deletion lld/MachO/ConcatOutputSection.cpp
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Expand Up @@ -306,7 +306,12 @@ void TextOutputSection::finalize() {
// contains several branch instructions in succession, then the distance
// from the current position to the position where the thunks are inserted
// grows. So leave room for a bunch of thunks.
unsigned slop = 256 * thunkSize;
#ifdef LLD_MACHO_SLOP_FACTOR
const unsigned kSlopFact = LLD_MACHO_SLOP_FACTOR;
#else
const unsigned kSlopFact = 256;
#endif
unsigned slop = kSlopFact * thunkSize;
while (finalIdx < endIdx) {
uint64_t expectedNewSize =
alignToPowerOf2(addr + size, inputs[finalIdx]->align) +
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