LLVM not optimizing out heap allocations when using a custom allocator #149230
Description
I tried this code:
#[global_allocator] static GLOBAL: MiMalloc = MiMalloc; #[unsafe(no_mangle)] pub fn test_heap_alloc_optimization(){ Box::new(4); }On Windows 11
I expected it to optimize out the heap allocation since the Box is not used anywhere, but the heap allocation is actually happening
Here is the assembly code.
test_heap_alloc_optimization: .seh_proc test_heap_alloc_optimization subq$40, %rsp .seh_stackalloc 40 .seh_endprologue callq _RNvCs5Yz3s5T3wso_7___rustc35___rust_no_alloc_shim_is_unstable_v2 movl$4, %ecx movl$4, %edx callq mi_malloc_aligned testq %rax, %rax je.LBB9_1 movl$4, (%rax) movq%rax, %rcx .seh_startepilogue addq$40, %rsp .seh_endepilogue jmpmi_free .LBB9_1: movl$4, %ecx movl$4, %edx callq _ZN5alloc5alloc18handle_alloc_error17h26b165d14d93d00aE int3 .seh_endprocI believe the allocation is happening at mi_maloc_aligned.
Here is what I got when mimalloc was removed from being the global allocator (so now its using default allocator)
test_heap_alloc_optimization: jmp_RNvCs5Yz3s5T3wso_7___rustc35___rust_no_alloc_shim_is_unstable_v2Now there is no heap allocation happening, as it should be.
Meta
rustc 1.93.0-nightly (6647be936 2025-11-09) binary: rustc commit-hash: 6647be93640686a2a443a49f15c3390b68c8b5dd commit-date: 2025-11-09 host: x86_64-pc-windows-msvc release: 1.93.0-nightly LLVM version: 21.1.3 On Linux (Debian)
I also tried it on linux, and still, heap allocation is happening
test_heap_alloc_optimization: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 callq *_RNvCsfHF5UXtEvMp_7___rustc35___rust_no_alloc_shim_is_unstable_v2@GOTPCREL(%rip) movl $4, %edi movl $4, %esi callq *mi_malloc_aligned@GOTPCREL(%rip) testq %rax, %rax je .LBB9_1 movl $4, (%rax) movq %rax, %rdi popq %rax .cfi_def_cfa_offset 8 jmpq *mi_free@GOTPCREL(%rip)I then tried it out with jemalloc, using the tikv-jemallocator crate
test_heap_alloc_optimization: .cfi_startproc pushq %rax .cfi_def_cfa_offset 16 callq*_RNvCsfHF5UXtEvMp_7___rustc35___rust_no_alloc_shim_is_unstable_v2@GOTPCREL(%rip) movl$4, %edi callq*_rjem_malloc@GOTPCREL(%rip) testq %rax, %rax je.LBB9_1 movl$4, (%rax) movl$4, %esi movq%rax, %rdi xorl %edx, %edx popq %rax .cfi_def_cfa_offset 8 jmpq*_rjem_sdallocx@GOTPCREL(%rip) .LBB9_1: .cfi_def_cfa_offset 16 movl$4, %edi movl$4, %esi callq*_ZN5alloc5alloc18handle_alloc_error17h2d947eb22614166fE@GOTPCREL(%rip) .Lfunc_end9: .size test_heap_alloc_optimization, .Lfunc_end9-test_heap_alloc_optimization .cfi_endprocStill, there is a heap allocation, at callq *_rjem_malloc@GOTPCREL(%rip) I believe
Meta
rustc 1.93.0-nightly (94b49fd99 2025-11-22) binary: rustc commit-hash: 94b49fd998d6723e0a9240a7cff5f9df37b84dd8 commit-date: 2025-11-22 host: x86_64-unknown-linux-gnu release: 1.93.0-nightly LLVM version: 21.1.5