[lld] [lld-macho]Define a macro to allow specifying the slop size at build time. (PR #164295)
Vy Nguyen via llvm-commits
llvm-commits at lists.llvm.org
Wed Oct 22 11:47:35 PDT 2025
https://github.com/oontvoo updated https://github.com/llvm/llvm-project/pull/164295
>From d6942973ad81b04e47f88d57c9d3fc96fb949b75 Mon Sep 17 00:00:00 2001
From: Vy Nguyen <vyng at google.com>
Date: Mon, 20 Oct 2025 14:17:17 -0400
Subject: [PATCH 1/2] [lld-macho]Define a macro to allow specifying the slop
size at build time.
We're seeing thunk size overrun in our large builds and it would be helpful to be able to increase the slop size.
However, this should probably not be changed between different runs of LLD, hence making it a build option so that it's fixed per lld binary.
---
lld/MachO/CMakeLists.txt | 5 +++++
lld/MachO/ConcatOutputSection.cpp | 7 ++++++-
2 files changed, 11 insertions(+), 1 deletion(-)
diff --git a/lld/MachO/CMakeLists.txt b/lld/MachO/CMakeLists.txt
index 3cd94ced75cc0..5ad12eb04f4c3 100644
--- a/lld/MachO/CMakeLists.txt
+++ b/lld/MachO/CMakeLists.txt
@@ -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)
+add_definitions("-DLLD_MACHO_SLOP_FACTOR=${LLD_MACHO_SLOP_FACTOR}")
+
set(LLVM_TARGET_DEFINITIONS Options.td)
tablegen(LLVM Options.inc -gen-opt-parser-defs)
add_public_tablegen_target(MachOOptionsTableGen)
diff --git a/lld/MachO/ConcatOutputSection.cpp b/lld/MachO/ConcatOutputSection.cpp
index 8067d23fa6faf..7a3476a145f0b 100644
--- a/lld/MachO/ConcatOutputSection.cpp
+++ b/lld/MachO/ConcatOutputSection.cpp
@@ -306,7 +306,12 @@ void TextOutputSection::finalize() {
// contains several branch instructions in succession, then the distance
// from the current position to the position where the thunks are inserted
// grows. So leave room for a bunch of thunks.
- unsigned slop = 256 * thunkSize;
+#ifdef LLD_MACHO_SLOP_FACTOR
+ const unsigned kSlopFact = LLD_MACHO_SLOP_FACTOR;
+#else
+ const unsigned kSlopFact = 256;
+#endif
+ unsigned slop = kSlopFact * thunkSize;
while (finalIdx < endIdx) {
uint64_t expectedNewSize =
alignToPowerOf2(addr + size, inputs[finalIdx]->align) +
>From f3ddb559fa7db14825e2b6cfbf6cb1b4d935c64e Mon Sep 17 00:00:00 2001
From: Vy Nguyen <vyng at google.com>
Date: Wed, 22 Oct 2025 14:32:46 -0400
Subject: [PATCH 2/2] make slop-scale a linker option instead of build-time
flag + update docs
---
lld/MachO/CMakeLists.txt | 5 -----
lld/MachO/ConcatOutputSection.cpp | 11 ++++-------
lld/MachO/Config.h | 1 +
lld/MachO/Driver.cpp | 9 +++++++++
lld/MachO/Options.td | 8 ++++++++
5 files changed, 22 insertions(+), 12 deletions(-)
diff --git a/lld/MachO/CMakeLists.txt b/lld/MachO/CMakeLists.txt
index 5ad12eb04f4c3..3cd94ced75cc0 100644
--- a/lld/MachO/CMakeLists.txt
+++ b/lld/MachO/CMakeLists.txt
@@ -1,8 +1,3 @@
-
-option(LLD_MACHO_SLOP_FACTOR
- "Specify the slop factor. For very large build, the default of 256 might be too small, which can cause thunk-range overrun. Recommend increasing this value to 1024 or higher as needed.", 256)
-add_definitions("-DLLD_MACHO_SLOP_FACTOR=${LLD_MACHO_SLOP_FACTOR}")
-
set(LLVM_TARGET_DEFINITIONS Options.td)
tablegen(LLVM Options.inc -gen-opt-parser-defs)
add_public_tablegen_target(MachOOptionsTableGen)
diff --git a/lld/MachO/ConcatOutputSection.cpp b/lld/MachO/ConcatOutputSection.cpp
index 7a3476a145f0b..e559676ef5e9b 100644
--- a/lld/MachO/ConcatOutputSection.cpp
+++ b/lld/MachO/ConcatOutputSection.cpp
@@ -306,12 +306,7 @@ 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.
-#ifdef LLD_MACHO_SLOP_FACTOR
- const unsigned kSlopFact = LLD_MACHO_SLOP_FACTOR;
-#else
- const unsigned kSlopFact = 256;
-#endif
- unsigned slop = kSlopFact * thunkSize;
+ unsigned slop = config->slopScale * thunkSize;
while (finalIdx < endIdx) {
uint64_t expectedNewSize =
alignToPowerOf2(addr + size, inputs[finalIdx]->align) +
@@ -389,7 +384,9 @@ void TextOutputSection::finalize() {
// above. If you hit this: For the current algorithm, just bumping up
// slop above and trying again is probably simplest. (See also PR51578
// comment 5).
- fatal(Twine(__FUNCTION__) + ": FIXME: thunk range overrun");
+ fatal(Twine(__FUNCTION__) +
+ ": FIXME: thunk range overrun. Consider increasing the "
+ "slop-scale with `--slop-scale=<unsigned_int>`.");
}
thunkInfo.isec =
makeSyntheticInputSection(isec->getSegName(), isec->getName());
diff --git a/lld/MachO/Config.h b/lld/MachO/Config.h
index a2ca5770bf952..759a8cbe3d353 100644
--- a/lld/MachO/Config.h
+++ b/lld/MachO/Config.h
@@ -224,6 +224,7 @@ struct Configuration {
bool disableVerify;
bool separateCstringLiteralSections;
bool tailMergeStrings;
+ unsigned slopScale = 256;
bool callGraphProfileSort = false;
llvm::StringRef printSymbolOrder;
diff --git a/lld/MachO/Driver.cpp b/lld/MachO/Driver.cpp
index 9b67db9fa55cf..dd481793e1655 100644
--- a/lld/MachO/Driver.cpp
+++ b/lld/MachO/Driver.cpp
@@ -1995,6 +1995,15 @@ bool link(ArrayRef<const char *> argsArr, llvm::raw_ostream &stdoutOS,
OPT_no_separate_cstring_literal_sections, false);
config->tailMergeStrings =
args.hasFlag(OPT_tail_merge_strings, OPT_no_tail_merge_strings, false);
+ if (auto *arg = args.getLastArg(OPT_slop_scale_eq)) {
+ StringRef v(arg->getValue());
+ unsigned slop = 0;
+ if (!llvm::to_integer(v, slop, 0))
+ error(arg->getSpelling() +
+ ": expected a non-negative integer, but got '" + arg->getValue() +
+ "'");
+ config->slopScale = slop;
+ }
auto IncompatWithCGSort = [&](StringRef firstArgStr) {
// Throw an error only if --call-graph-profile-sort is explicitly specified
diff --git a/lld/MachO/Options.td b/lld/MachO/Options.td
index be1a1cc2963d9..188dc0a501552 100644
--- a/lld/MachO/Options.td
+++ b/lld/MachO/Options.td
@@ -1095,6 +1095,14 @@ defm tail_merge_strings
: BB<"tail-merge-strings", "Enable string tail merging",
"Disable string tail merging to improve link-time performance">,
Group<grp_rare>;
+def slop_scale_eq
+ : Joined<["--"], "slop_scale=">,
+ MetaVarName<"<unsigned_int>">,
+ HelpText<"Specify the slop scale. Default value is 256. If your binary "
+ "has too many consec branch instructions resulting in "
+ "thunk-range overrun, then you need to increase this value to a "
+ "higher value, such as 512 or 1024, etc">,
+ Group<grp_rare>;
def grp_deprecated : OptionGroup<"deprecated">, HelpText<"DEPRECATED">;
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