[llvm-bugs] [Bug 42099] New: Clang inserts a call to memset when compiling with -nostdlib

via llvm-bugs llvm-bugs at lists.llvm.org
Sat Jun 1 10:17:32 PDT 2019 

https://bugs.llvm.org/show_bug.cgi?id=42099

            Bug ID: 42099
           Summary: Clang inserts a call to memset when compiling with
                    -nostdlib
           Product: clang
           Version: 7.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: C
          Assignee: unassignedclangbugs at nondot.org
          Reporter: jynelson at email.sc.edu
                CC: blitzrakete at gmail.com, dgregor at apple.com,
                    erik.pilkington at gmail.com, llvm-bugs at lists.llvm.org,
                    richard-llvm at metafoo.co.uk

Created attachment 22062
  --> https://bugs.llvm.org/attachment.cgi?id=22062&action=edit
The pre-processed source file (as minimal as I could make it)

Clang incorrectly optimizes a function call equivalent to memset to a runtime
call to memset even when compiling with -nostdlib. While normally this would be
a useful optimization, in this case it causes compilation to fail.

The bad optimization disappears when compiling without optimization. I have
also been able to replicate the problem on clang 6.0.

See also the corrosponding GCC bug:
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90701.

Output of clang-7 -v -save-temps -nostdlib -O3 -Wall repro2.c:
clang version 7.0.0-3~ubuntu0.18.04.1 (tags/RELEASE_700/final)
Target: x86_64-pc-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation: /usr/bin/../lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/7.4.0
Found candidate GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/i686-linux-gnu/8
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/7.4.0
Found candidate GCC installation: /usr/lib/gcc/x86_64-linux-gnu/8
Selected GCC installation: /usr/bin/../lib/gcc/x86_64-linux-gnu/8
Candidate multilib: .;@m64
Candidate multilib: 32;@m32
Candidate multilib: x32;@mx32
Selected multilib: .;@m64
 "/usr/lib/llvm-7/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -E
-save-temps=cwd -disable-free -disable-llvm-verifier -discard-value-names
-main-file-name repro2.c -mrelocation-model static -mthread-model posix
-fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables
-fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb
-momit-leaf-frame-pointer -v -resource-dir /usr/lib/llvm-7/lib/clang/7.0.0
-internal-isystem /usr/local/include -internal-isystem
/usr/lib/llvm-7/lib/clang/7.0.0/include -internal-externc-isystem
/usr/include/x86_64-linux-gnu -internal-externc-isystem /include
-internal-externc-isystem /usr/include -O3 -Wall -fdebug-compilation-dir
/usr/local/src/proot-rs/src/kernel/execve/loader -ferror-limit 19
-fmessage-length 93 -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -vectorize-loops -vectorize-slp -o repro2.i -x c repro2.c
-faddrsig
clang -cc1 version 7.0.0 based upon LLVM 7.0.0 default target
x86_64-pc-linux-gnu
ignoring nonexistent directory "/include"
#include "..." search starts here:
#include <...> search starts here:
 /usr/local/include
 /usr/lib/llvm-7/lib/clang/7.0.0/include
 /usr/include/x86_64-linux-gnu
 /usr/include
End of search list.
 "/usr/lib/llvm-7/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -emit-llvm-bc
-emit-llvm-uselists -save-temps=cwd -disable-free -disable-llvm-verifier
-discard-value-names -main-file-name repro2.c -mrelocation-model static
-mthread-model posix -fmath-errno -masm-verbose -mconstructor-aliases
-munwind-tables -fuse-init-array -target-cpu x86-64 -dwarf-column-info
-debugger-tuning=gdb -momit-leaf-frame-pointer -v -resource-dir
/usr/lib/llvm-7/lib/clang/7.0.0 -O3 -Wall -fdebug-compilation-dir
/usr/local/src/proot-rs/src/kernel/execve/loader -ferror-limit 19
-fmessage-length 93 -fobjc-runtime=gcc -fdiagnostics-show-option
-fcolor-diagnostics -vectorize-loops -vectorize-slp -disable-llvm-passes -o
repro2.bc -x cpp-output repro2.i -faddrsig
clang -cc1 version 7.0.0 based upon LLVM 7.0.0 default target
x86_64-pc-linux-gnu
#include "..." search starts here:
End of search list.
 "/usr/lib/llvm-7/bin/clang" -cc1 -triple x86_64-pc-linux-gnu -S
-save-temps=cwd -disable-free -disable-llvm-verifier -discard-value-names
-main-file-name repro2.c -mrelocation-model static -mthread-model posix
-fmath-errno -masm-verbose -mconstructor-aliases -munwind-tables
-fuse-init-array -target-cpu x86-64 -dwarf-column-info -debugger-tuning=gdb
-momit-leaf-frame-pointer -v -resource-dir /usr/lib/llvm-7/lib/clang/7.0.0 -O3
-Wall -fdebug-compilation-dir /usr/local/src/proot-rs/src/kernel/execve/loader
-ferror-limit 19 -fmessage-length 93 -fobjc-runtime=gcc
-fdiagnostics-show-option -fcolor-diagnostics -vectorize-loops -vectorize-slp
-o repro2.s -x ir repro2.bc -faddrsig
clang -cc1 version 7.0.0 based upon LLVM 7.0.0 default target
x86_64-pc-linux-gnu
 "/usr/lib/llvm-7/bin/clang" -cc1as -triple x86_64-pc-linux-gnu -filetype obj
-main-file-name repro2.c -target-cpu x86-64 -dwarf-version=4 -mrelocation-model
static -o repro2.o repro2.s
 "/usr/bin/ld" -z relro --hash-style=gnu --eh-frame-hdr -m elf_x86_64
-dynamic-linker /lib64/ld-linux-x86-64.so.2 -o a.out
-L/usr/bin/../lib/gcc/x86_64-linux-gnu/8
-L/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu
-L/lib/x86_64-linux-gnu -L/lib/../lib64 -L/usr/lib/x86_64-linux-gnu
-L/usr/bin/../lib/gcc/x86_64-linux-gnu/8/../../.. -L/usr/lib/llvm-7/bin/../lib
-L/lib -L/usr/lib repro2.o
repro2.o: In function `_start':
repro2.c:(.text+0x50): undefined reference to `memset'
repro2.c:(.text+0x5d): undefined reference to `memset'
repro2.c:(.text+0x7e): undefined reference to `memset'
clang: error: linker command failed with exit code 1 (use -v to see invocation)

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