The iOS simulator can run your app in 32 and 64-bit modes. This allows you to work out a lot of 64-bit issues and make sure it is ready for both armv7 and arm64. To do this, it compiles your app for i386 and x86_64 and requires the libraries for both architectures.
A std::array<int, 3> is 96 bits wide on your implementation. As such the ABI declares that it should be returned in RAX + the low 32 bits of RDX (aka EDX). 4294967296 is 232, in hex it is $1’0000’0000. So the movabs stores 0 in the low order 32 bits of RAX, and 1 in … Read more
movq (assuming you’re talking about x86) is a move of a quadword (64-bit value). This particular instruction: movq (%rsp), %rsp looks very much like code that will walk up through stack frames. This particular instruction grabs the quadword pointed to by the current stack pointer, and loads it into the stack pointer, overwriting it. By … Read more
General part EDIT: Linux irrelevant parts removed While not totally wrong, narrowing down to int 0x80 and syscall oversimplifies the question as with sysenter there is at least a 3rd option. Using 0x80 and eax for syscall number, ebx, ecx, edx, esi, edi, and ebp to pass parameters is just one of many possible other … Read more
Alignment optimisations 1. Use .p2align <abs-expr> <abs-expr> <abs-expr> instead of align. Grants fine-grained control using its 3 params param1 – Align to what boundary. param2 – Fill padding with what (zeroes or NOPs). param3 – Do NOT align if padding would exceed specified number of bytes. 2. Align the start of a frequently used code … Read more
No, you can’t. Both 16-bit and 32-bit Windows lived in a 32-bit linear address space. The terms 16 and 32 refer to the size of the offset relative to the selector. … First, notice that a full-sized 16-bit pointer and a 32-bit flat pointer are the same size. The value 0x0123:0x467 requires 32 bits, and … Read more
Given these two quotes, is the red zone above the stacked return address or below the stacked return address? The red zone is the 128 bytes just below rsp, i.e. rsp – 128 to rsp – 1. Since this red zone is relative to RSP, does it move downward with each push and does it … Read more
@fd – the RedHat bug had your answer. The mallinfo function has been deprecated, and won’t be updated. A true query stats API is TDB. Today, you have malloc_stats and malloc_info. I can’t find any documentation on either one, but here’s what they give you. Is this close enough to what you need? (gdb) call … Read more
Yes, GCC generally avoids writing to partial registers, unless optimizing for size (-Os) instead of purely speed (-O3). Some cases require writing at least the 32-bit register for correctness, so a better example would be something like: char foo(char *p) { return *p; } compiles to movzx eax, byte ptr [rdi] instead of mov al, … Read more
Can’t we multiply rax with edi directly? We can’t imul rax, rdi because the calling convention allows the caller to leave garbage in the high bits of RDI; only the EDI part contains the value. This is a non-issue when inlining; writing a 32-bit register does implicitly zero-extend to the full 64-bit register, so the … Read more