This optimization has since been implemented in GCC. It can be enabled with the -fno-plt option and the noplt function attribute: Do not use the PLT for external function calls in position-independent code. Instead, load the callee address at call sites from the GOT and branch to it. This leads to more efficient code by … Read more
The visibility(“hidden”) attribute does not suppress a symbol from an object file and cannot prevent a symbol being extracted by nm. It just instructs the dynamic linker that the symbol cannot be called from outside a shared library that contains it. Consider a source file file.c containing your example functions: int f_b1(){ return 21 ; … Read more
The C standard doesn’t say anything about “sections” in the sense that you mean, so you’ll need to use extensions specific to your compiler. With GCC, you will want to use the section attribute: extern void foobar(void) __attribute__((section(“bar”))); There is some limited documentation here, including a warning: Some file formats do not support arbitrary sections … Read more
After that, however, it subtracted (dividend/2^31) from the result. Actually, it subtracts dividend >> 31, which is -1 for negative dividend, and 0 for non-negative dividend, when right-shifting negative integers is arithmetic right-shift (and int is 32 bits wide). 0x6666667 = (2^34 + 6)/10 So for x < 0, we have, writing x = 10*k … Read more
The following example works for me: $ dd if=/dev/urandom of=binblob bs=1024k count=1 $ objcopy -I binary -O elf32-little binblob binblob.o $ file binblob.o binblob.o: ELF 32-bit LSB relocatable, no machine, version 1 (SYSV), not stripped $ nm -S -t d binblob.o 0000000001048576 D _binary_binblob_end 0000000001048576 A _binary_binblob_size 0000000000000000 D _binary_binblob_start I.e. no need to specify … Read more
I had a response on the cfe-dev (clang front end developers’ list) from one of the developers: LLVM currently always spills “rm” constraints in order to simplify the handling of inline asm in the backend (you can ask on llvmdev if you want details). I don’t know of any plans to fix this in the … Read more
Use gcc -E to only run the preprocessor part, e.g. give a file in.c #if 0 0; #endif #if 1 1; #endif running $ gcc -E in.c -o in.i yields a file in.i # 1 “in.cpp” # 1 “<built-in>” # 1 “<command-line>” # 1 “in.cpp” 1; i.e. the parts behind the #if 0 got removed. … Read more
A simpler solution that fixed this problem for me was adding -Wa,-q to the compiler flags. From the man pages for as (version 1.38): -q Use the clang(1) integrated assembler instead of the GNU based system assembler. The -Wa part passes it from the compiler driver to the assembler, much like -Wl passes arguments … Read more
Second question first: div is a very slow instruction (more than 20 clock cycles). The sequence above consists of more instructions, but they’re all relatively fast, so it’s a net win in terms of speed. The first five instructions (up to and including the shrl) compute i/10 (I’ll explain how in a minute). The next … Read more
In the case of rpath, it makes no sense to use a relative path, since a relative path will be relative to the current working directory, NOT relative to the directory where the binary/library was found. So it simply won’t work for executables found in $PATH or libraries in pretty much any case. Instead, you … Read more