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[libunwind] Optimize dl_iterate_phdr's findUnwindSectionsByPhdr

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Currently, findUnwindSectionsByPhdr is slightly micro-optimized for the
case where the first callback has the target address, and is otherwise
very inefficient -- it decodes .eh_frame_hdr even when no PT_LOAD
matches the PC. (If the FrameHeaderCache is enabled, then the
micro-optimization only helps the first time unwind info is looked up.)

Instead, it makes more sense to optimize for the case where the
callback *doesn't* find the target address, so search for a PT_LOAD
segment first, and only look for the unwind info section if a matching
PT_LOAD is found.

This change helps on an Android benchmark with 100 shared objects,
where the DSO at the end of the dl_iterate_phdr list throws 10000
exceptions. Assuming the frame cache is disabled, this change cuts
about 30-40% off the benchmark's runtime.

Reviewed By: compnerd, saugustine, #libunwind

Differential Revision: https://reviews.llvm.org/D87881
This commit is contained in:
Ryan Prichard 2020-09-23 14:25:23 -07:00
parent b16d6653c0
commit 881aba7071
1 changed files with 27 additions and 19 deletions
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46
libunwind/src/AddressSpace.hpp
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@ -462,29 +462,37 @@ static int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo,
#endif
Elf_Addr image_base = calculateImageBase(pinfo);
// Most shared objects seen in this callback function likely don't contain the
// target address, so optimize for that. Scan for a matching PT_LOAD segment
// first and bail when it isn't found.
bool found_text = false;
bool found_unwind = false;
// Third phdr is usually the executable phdr.
if (pinfo->dlpi_phnum > 2)
found_text = checkAddrInSegment(&pinfo->dlpi_phdr[2], image_base, cbdata);
// PT_GNU_EH_FRAME and PT_ARM_EXIDX are usually near the end. Iterate
// backward. We already know that there is one or more phdrs.
for (Elf_Half i = pinfo->dlpi_phnum; i > 0; i--) {
const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i - 1];
if (!found_unwind && checkForUnwindInfoSegment(phdr, image_base, cbdata))
found_unwind = true;
else if (!found_text && checkAddrInSegment(phdr, image_base, cbdata))
for (Elf_Half i = 0; i < pinfo->dlpi_phnum; ++i) {
if (checkAddrInSegment(&pinfo->dlpi_phdr[i], image_base, cbdata)) {
found_text = true;
if (found_text && found_unwind) {
#if defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
TheFrameHeaderCache.add(cbdata->sects);
#endif
return 1;
break;
}
}
return 0;
if (!found_text)
return 0;
// PT_GNU_EH_FRAME and PT_ARM_EXIDX are usually near the end. Iterate
// backward.
bool found_unwind = false;
for (Elf_Half i = pinfo->dlpi_phnum; i > 0; i--) {
const Elf_Phdr *phdr = &pinfo->dlpi_phdr[i - 1];
if (checkForUnwindInfoSegment(phdr, image_base, cbdata)) {
found_unwind = true;
break;
}
}
if (!found_unwind)
return 0;
#if defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE)
TheFrameHeaderCache.add(cbdata->sects);
#endif
return 1;
}
#endif // defined(_LIBUNWIND_USE_DL_ITERATE_PHDR)