[cfe-dev] How to debug if LTO generate wrong code?

[Shi, Steven via cfe-dev]

Hello,
I'm enabling clang LTO to improve code size of Uefi standard (http://www.uefi.org/) firmware (https://github.com/tianocore/edk2), which is mostly C code. My project is in https://github.com/shijunjing/edk2 branch llvm : https://github.com/shijunjing/edk2/tree/llvm. I find my most firmware modules work well after enable LTO, but some X64 modules will not run (e.g. hang with CPU exception) , and these X64 modules work well if build with the LTO disabled (-fno-lto).
I don't know how to efficiently debug these LTO wrong code and investigate if there is compiler's bug. I appreciate if anyone can  give me some suggestions about the clang LTO issue debug method, commands, or BKMs.

Below are  my clang LTO build tools and options, I use clang 3.8 release with binutils 2.26 ld (I've pushed ld support LLVM gold plugin https://sourceware.org/bugzilla/show_bug.cgi?id=20070). Any suggestion is welcome!



##################
# CLANGLTO38 X64 definitions
##################
*_CLANGLTO38_X64_OBJCOPY_PATH         = DEF(GCC53_X64_PREFIX)objcopy
*_CLANGLTO38_X64_CC_PATH              = DEF(CLANG38_X64_PREFIX)clang
*_CLANGLTO38_X64_SLINK_PATH           = DEF(CLANG38_X64_PREFIX)llvm-ar
*_CLANGLTO38_X64_DLINK_PATH           = DEF(CLANG38_X64_PREFIX)clang
*_CLANGLTO38_X64_ASM_PATH             = DEF(CLANG38_X64_PREFIX)clang
*_CLANGLTO38_X64_PP_PATH              = DEF(CLANG38_X64_PREFIX)clang
*_CLANGLTO38_X64_RC_PATH              = DEF(GCC53_X64_PREFIX)objcopy

*_CLANGLTO38_X64_CC_FLAGS = -c -fshort-wchar -fno-strict-aliasing -Wall -Werror -Wno-array-bounds -Wno-empty-body -ffunction-sections -fdata-sections -include AutoGen.h -DSTRING_ARRAY_NAME=$(BASE_NAME)Strings -fno-stack-protector -fno-builtin -mms-bitfields -Wno-address -Wno-shift-negative-value -Wno-parentheses-equality -Wno-unknown-pragmas -Wno-tautological-constant-out-of-range-compare -Wno-incompatible-library-redeclaration -target x86_64-pc-linux-gnu -fno-asynchronous-unwind-tables -m64 -Wno-enum-conversion "-DEFIAPI=__attribute__((ms_abi))" -mno-red-zone -mcmodel=large -g -Os -flto
*_CLANGLTO38_X64_DLINK_FLAGS  = -flto -nostdlib -Wl,-n -Wl,-q -Wl,--gc-sections -Wl,-z,common-page-size=0x40 -Wl,--entry,$(IMAGE_ENTRY_POINT) -Wl,-u,$(IMAGE_ENTRY_POINT) -Wl,-Map,$(DEST_DIR_DEBUG)/$(BASE_NAME).map -Wl,-melf_x86_64 -Wl,--oformat=elf64-x86-64
*_CLANGLTO38_X64_ASM_FLAGS            = -c -x assembler -imacros $(DEST_DIR_DEBUG)/AutoGen.h -m64 -target x86_64-pc-linux-gnu
*_CLANGLTO38_X64_RC_FLAGS             = -I binary -O elf64-x86-64        -B i386    --rename-section .data=.hii
*_CLANGLTO38_X64_NASM_FLAGS           = -f elf64



Steven Shi
Intel\SSG\STO\UEFI Firmware

Tel: +86 021-61166522
iNet: 821-6522

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