compiler-optimization

问题 I'm trying to create as small ELF as possible. I created a test file like this (NASM syntax): SECTION .text dd 0xdeadbeef With this linker script: SECTIONS { .text : { *(.text) } } Then I checked sizes of flat binary and ELFs built two ways: nasm -f bin -o test test.asm It's flat binary, so 4 bytes. nasm -f elf -o test.o test.asm i686-elf-ld -Tlinker.ld test.o -o test I'd expect something like 500 bytes max, but the resulting file is 4396 bytes long! There is an option however, named --strip

问题 I'm trying to create as small ELF as possible. I created a test file like this (NASM syntax): SECTION .text dd 0xdeadbeef With this linker script: SECTIONS { .text : { *(.text) } } Then I checked sizes of flat binary and ELFs built two ways: nasm -f bin -o test test.asm It's flat binary, so 4 bytes. nasm -f elf -o test.o test.asm i686-elf-ld -Tlinker.ld test.o -o test I'd expect something like 500 bytes max, but the resulting file is 4396 bytes long! There is an option however, named --strip

问题 I'm trying to create as small ELF as possible. I created a test file like this (NASM syntax): SECTION .text dd 0xdeadbeef With this linker script: SECTIONS { .text : { *(.text) } } Then I checked sizes of flat binary and ELFs built two ways: nasm -f bin -o test test.asm It's flat binary, so 4 bytes. nasm -f elf -o test.o test.asm i686-elf-ld -Tlinker.ld test.o -o test I'd expect something like 500 bytes max, but the resulting file is 4396 bytes long! There is an option however, named --strip

问题 Consider the following code: let u: Vec<u8> = (64..74).collect(); let v: Vec<u8> = u.iter().map(|i| i + 1).collect(); u was not moved, therefore v was inevitably newly allocated. But if I do the following: let w: Vec<u8> = u.into_iter().map(|i| i + 1).collect(); u was moved and w is the name of its transformation. Here is some pseudo-code representing what I mean: mark u as "moved" for i = 0..10: u[i] += 1 w = u There is (in my opinion) no need for a new allocation, since we map a type to

问题 I looked at this question trying to better understand @tailrec annotation in scala. What I'm not sure is whether the annotation also hints the compiler to do some optimizations or it's only used for warnings when you mark a method that is not a tail-recursion? More specifically - is this annotation might affect performance in any way? For example, if I don't put this annotation, the compiler will compile a tail-recursive function as non-tail recursive? 回答1: As per the scaladoc: A method

问题 I was benchmarking some counting in a loop code. g++ was used with -O2 code and I noticed that it has some perf problems when some condition is true in 50% of the cases. I assumed that may mean that code does unnecessary jumps(since clang produces faster code so it is not some fundamental limitation). What I find in this asm output funny is that code jumps over one simple add. => 0x42b46b <benchmark_many_ints()+1659>: movslq (%rdx),%rax 0x42b46e <benchmark_many_ints()+1662>: mov %rax,%rcx

问题 I was benchmarking some counting in a loop code. g++ was used with -O2 code and I noticed that it has some perf problems when some condition is true in 50% of the cases. I assumed that may mean that code does unnecessary jumps(since clang produces faster code so it is not some fundamental limitation). What I find in this asm output funny is that code jumps over one simple add. => 0x42b46b <benchmark_many_ints()+1659>: movslq (%rdx),%rax 0x42b46e <benchmark_many_ints()+1662>: mov %rax,%rcx

问题 Can a reasonable decent compiler discard this const static variable class A{ const static int a = 3; } if it is nowhere used in the compiled binary or does it show up anyway in the binary? 回答1: Short answer: Maybe. The standard does not say the compiler HAS to keep the constants (or strings, or functions, or anything else), if it's never used. Long answer: It very much depends on the circumstances. If the compiler can clearly determine that it is not used, it will remove unused constants. If

问题 Can a reasonable decent compiler discard this const static variable class A{ const static int a = 3; } if it is nowhere used in the compiled binary or does it show up anyway in the binary? 回答1: Short answer: Maybe. The standard does not say the compiler HAS to keep the constants (or strings, or functions, or anything else), if it's never used. Long answer: It very much depends on the circumstances. If the compiler can clearly determine that it is not used, it will remove unused constants. If

问题 Can a reasonable decent compiler discard this const static variable class A{ const static int a = 3; } if it is nowhere used in the compiled binary or does it show up anyway in the binary? 回答1: Short answer: Maybe. The standard does not say the compiler HAS to keep the constants (or strings, or functions, or anything else), if it's never used. Long answer: It very much depends on the circumstances. If the compiler can clearly determine that it is not used, it will remove unused constants. If