compiler-optimization

问题 As far as compiler optimizations go, is it legal and/or possible to change a heap allocation to a stack allocation? Or would that break the as-if rule? For example, say this is the original version of the code { Foo* f = new Foo(); f->do_something(); delete f; } Would a compiler be able to change this to the following { Foo f{}; f.do_something(); } I wouldn't think so, because that would have implications if the original version was relying on things like custom allocators. Does the standard

问题 This is a simple question but I just came across it. In the code snippet below I create three pointers. I know the three will exhibit equivalent behavior (all point to the same thing), but I honestly thought the third action in the code was the most "efficient", meaning that it would generate less assembly instructions to accomplish the same thing as the other two. I assumed that the first two have to first deference a pointer, and then take the memory address of the thing that was

问题 This is a simple question but I just came across it. In the code snippet below I create three pointers. I know the three will exhibit equivalent behavior (all point to the same thing), but I honestly thought the third action in the code was the most "efficient", meaning that it would generate less assembly instructions to accomplish the same thing as the other two. I assumed that the first two have to first deference a pointer, and then take the memory address of the thing that was

问题 Recently I had a discussion about someone who wanted to check for signed int overflow like this if (A + B < 2 * max(A, B)) . Lets ignore for a second that the logic itself is wrong and discuss signed integer overflow in context of C/C++. (Which I believe fully inherits this part of standard from C). What kinds of check that need signed integer overflow will be optimized away by current-ish GCC and which won't? Since the original text wasn't all that well formulated and apparently

问题 Recently I had a discussion about someone who wanted to check for signed int overflow like this if (A + B < 2 * max(A, B)) . Lets ignore for a second that the logic itself is wrong and discuss signed integer overflow in context of C/C++. (Which I believe fully inherits this part of standard from C). What kinds of check that need signed integer overflow will be optimized away by current-ish GCC and which won't? Since the original text wasn't all that well formulated and apparently

问题 I saw somewhere that the GCC compiler might prefer sometimes not using conditional mov when converting my code into ASM. What are the cases where it might choose to do something other than conditional mov? 回答1: Compilers often favour if-conversion to cmov when both sides of the branch are short, especially with a ternary so you always assign a C variable. e.g. if(x) y=bar; sometimes doesn't optimize to CMOV but y = x ? bar : y; does use CMOV more often. Especially when y is an array entry

问题 I saw somewhere that the GCC compiler might prefer sometimes not using conditional mov when converting my code into ASM. What are the cases where it might choose to do something other than conditional mov? 回答1: Compilers often favour if-conversion to cmov when both sides of the branch are short, especially with a ternary so you always assign a C variable. e.g. if(x) y=bar; sometimes doesn't optimize to CMOV but y = x ? bar : y; does use CMOV more often. Especially when y is an array entry

问题 Basically I am trying to understand why both gcc/clang use xmm register for their __builtin_memset even when the memory destination and size are both divisible by sizeof ymm (or zmm for that matter) and the CPU supports AVX2 / AVX512 . and why GCC implements __builtin_memset on medium sized values without any SIMD (again assuming CPU supports SIMD). For example: __builtin_memset(__builtin_assume_aligned(ptr, 64), -1, 64)); Will compile to: vpcmpeqd %xmm0, %xmm0, %xmm0 vmovdqa %xmm0, (%rdi)

问题 I'm reading through the source code of a library (QNNPack) and noticed this line (https://github.com/pytorch/QNNPACK/blob/24d57f21503ba8ab0f8bb5d24148754a91266b9c/src/q8gemm/6x4-neon.c#L23): void funcName(..., const union some_union_type some_union_arg[restrict static 1]) { // ... } I understand the keyword restrict and static in general, but I'm afraid I do not know the reason behind this. I didn't find anything on Google, perhaps I searched wrong. I'm guessing here it's a way of telling the

问题 I'm reading through the source code of a library (QNNPack) and noticed this line (https://github.com/pytorch/QNNPACK/blob/24d57f21503ba8ab0f8bb5d24148754a91266b9c/src/q8gemm/6x4-neon.c#L23): void funcName(..., const union some_union_type some_union_arg[restrict static 1]) { // ... } I understand the keyword restrict and static in general, but I'm afraid I do not know the reason behind this. I didn't find anything on Google, perhaps I searched wrong. I'm guessing here it's a way of telling the