[lldb][Darwin] Read Mach-O binaries out of memory more efficiently#200072
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jasonmolenda merged 3 commits intoMay 27, 2026
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When lldb needs to read a Mach-O binary out of memory, it first reads 512 bytes to get the mach header, which includes the size of the load commands, and then does a second read to get the mach header and load commands. I am changing the initial read to get 3192 bytes, which will include the full load commands for most binaries. In April I changed debugserver to return the correct size of the mach header and load commands in a `sizeof_mh_and_loadcmds` key. If this number is provided, refine the amount we read to this size. This reduces the number of memory read packets we issue from 2 to 1 for a memory module, outside of packets that may be needed to get the symbol table.
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@llvm/pr-subscribers-lldb Author: Jason Molenda (jasonmolenda) ChangesWhen lldb needs to read a Mach-O binary out of memory, it first reads 512 bytes to get the mach header, which includes the size of the load commands, and then does a second read to get the mach header and load commands. I am changing the initial read to get 3192 bytes, which will include the full load commands for most binaries. In April I changed debugserver to return the correct size of the mach header and load commands in a This reduces the number of memory read packets we issue from 2 to 1 for a memory module, outside of packets that may be needed to get the symbol table. Full diff: https://github.com/llvm/llvm-project/pull/200072.diff 1 Files Affected:
diff --git a/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderDarwin.cpp b/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderDarwin.cpp
index f666795c25860..65516cdb2ac47 100644
--- a/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderDarwin.cpp
+++ b/lldb/source/Plugins/DynamicLoader/MacOSX-DYLD/DynamicLoaderDarwin.cpp
@@ -177,7 +177,7 @@ ModuleSP DynamicLoaderDarwin::FindTargetModuleForImageInfo(
// added to the target, don't let it be called for every one.
if (!module_sp || module_sp->GetObjectFile() == nullptr) {
llvm::Expected<ModuleSP> module_sp_or_err = m_process->ReadModuleFromMemory(
- image_info.file_spec, image_info.address);
+ image_info.file_spec, image_info.address, image_info.mh_and_load_cmd_size);
if (auto err = module_sp_or_err.takeError()) {
LLDB_LOG_ERROR(GetLog(LLDBLog::DynamicLoader), std::move(err),
"Failed to load module from memory: {0}");
@@ -445,6 +445,9 @@ bool DynamicLoaderDarwin::JSONImageInformationIntoImageInfo(
mh->GetValueForKey("cpusubtype")->GetUnsignedIntegerValue();
image_infos[i].header.filetype =
mh->GetValueForKey("filetype")->GetUnsignedIntegerValue();
+ if (mh->HasKey("sizeof_mh_and_loadcmds"))
+ image_infos[i].mh_and_load_cmd_size =
+ mh->GetValueForKey("sizeof_mh_and_loadcmds")->GetUnsignedIntegerValue();
if (image->HasKey("min_version_os_name")) {
std::string os_name =
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✅ With the latest revision this PR passed the C/C++ code formatter. |
| mh->GetValueForKey("cpusubtype")->GetUnsignedIntegerValue(); | ||
| image_infos[i].header.filetype = | ||
| mh->GetValueForKey("filetype")->GetUnsignedIntegerValue(); | ||
| if (mh->HasKey("sizeof_h_and_loadcmds")) |
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| if (mh->HasKey("sizeof_h_and_loadcmds")) | |
| if (mh->HasKey("sizeof_mh_and_loadcmds")) |
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…lvm#200072) When lldb needs to read a Mach-O binary out of memory, it first reads 512 bytes to get the mach header, which includes the size of the load commands, and then does a second read to get the mach header and load commands. I am changing the initial read to get 3192 bytes, which will include the full load commands for most binaries. In April I changed debugserver to return the correct size of the mach header and load commands in a `sizeof_mh_and_loadcmds` key. If this number is provided, refine the amount we read to this size. This reduces the number of memory read packets we issue from 2 to 1 for a memory module, outside of packets that may be needed to get the symbol table. (cherry picked from commit 9409c07)
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When lldb needs to read a Mach-O binary out of memory, it first reads 512 bytes to get the mach header, which includes the size of the load commands, and then does a second read to get the mach header and load commands.
I am changing the initial read to get 3192 bytes, which will include the full load commands for most binaries.
In April I changed debugserver to return the correct size of the mach header and load commands in a
sizeof_mh_and_loadcmdskey. If this number is provided, refine the amount we read to this size.This reduces the number of memory read packets we issue from 2 to 1 for a memory module, outside of packets that may be needed to get the symbol table.