From 881aba7071c6e4cc2417e875ca5027ec7c0a92a3 Mon Sep 17 00:00:00 2001 From: Ryan Prichard Date: Wed, 23 Sep 2020 14:25:23 -0700 Subject: [PATCH] [libunwind] Optimize dl_iterate_phdr's findUnwindSectionsByPhdr 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 --- libunwind/src/AddressSpace.hpp | 38 +++++++++++++++++++++++--------------- 1 file changed, 23 insertions(+), 15 deletions(-) diff --git a/libunwind/src/AddressSpace.hpp b/libunwind/src/AddressSpace.hpp index 7a926ec..171318f 100644 --- a/libunwind/src/AddressSpace.hpp +++ b/libunwind/src/AddressSpace.hpp @@ -462,29 +462,37 @@ static int findUnwindSectionsByPhdr(struct dl_phdr_info *pinfo, #endif Elf_Addr image_base = calculateImageBase(pinfo); - 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); + // 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; + for (Elf_Half i = 0; i < pinfo->dlpi_phnum; ++i) { + if (checkAddrInSegment(&pinfo->dlpi_phdr[i], image_base, cbdata)) { + found_text = true; + break; + } + } + if (!found_text) + return 0; // 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. + // 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 (!found_unwind && checkForUnwindInfoSegment(phdr, image_base, cbdata)) + if (checkForUnwindInfoSegment(phdr, image_base, cbdata)) { found_unwind = true; - else if (!found_text && checkAddrInSegment(phdr, 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_unwind) + return 0; + +#if defined(_LIBUNWIND_USE_FRAME_HEADER_CACHE) + TheFrameHeaderCache.add(cbdata->sects); +#endif + return 1; } #endif // defined(_LIBUNWIND_USE_DL_ITERATE_PHDR) -- 2.7.4