tl;dr - alloca costs, history, and why it is problematic Alloca is somewhat more expensive on x86/x64 than a single instruction. [0] shows the code generation

tl;dr - alloca costs, history, and why it is problematic Alloca is somewhat more expensive on x86/x64 than a single instruction. [0] shows the code generation for four functions that generate and sum an iota array. I used -O1 to make the differences more apparent. iota_sum_alloca and iota_sum_vla generate similar code. They both require a frame pointer (RBP) and code to preserve the 16 byte alignment of the stack frame. iota_sum_const_alloca and iota_sum_array generate identical code. Clang recognizes that alloca is invoked with a constant argument. History of Alloca Alloca was originally written for unix V7 [1]. Doug Gwyn wrote a public domain implementation [2] in the early 80s for porting existing programs. The FSF used Gwyn's alloca implementation in GDB, Emacs, and other programs. This helped to spread the idea. Problems of Alloca [3] is a comp.compilers thread that discusses some of the issues with alloca. Linus does not want either VLAs or alloca in the Linux kernel [4]. References: [0] https://godbolt.org/g/1JyXhQ https://godbolt.org/g/1JyXhQ [1] http://yarchive.net/comp/alloca.html http://yarchive.net/comp/alloca.html [2] https://github.com/darchons/android-gdb/blob/android-gdb_7_5/gdb/gnulib/import/alloca.c https://github.com/darchons/android-gdb/blob/android-gdb_7_5... [3] http://compilers.iecc.com/comparch/article/91-12-079 http://compilers.iecc.com/comparch/article/91-12-079 [4] https://groups.google.com/forum/#!msg/fa.linux.kernel/ROgkTBO4VYI/sQZN3R9bEV4J https://groups.google.com/forum/#!msg/fa.linux.kernel/ROgkTB... Edited for minor formatting changes.