https://llvm.org/bugs/show_bug.cgi?id=30862
Bug ID: 30862
Summary: WebAssembly alias stack pointer not initialized
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Backend: WebAssembly
Assignee: unassignedb...@nondot.org
Reporter: timo.sav...@iki.fi
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 17529
--> https://llvm.org/bugs/attachment.cgi?id=17529&action=edit
Small demo program
LLVM seems to use the 32-bit value at WebAssembly linear memory offset 4 as
__stack_pointer, which is used to access the secondary stack within the linear
memory. However, while it's correctly maintained by function prologue and
epilogue, it's not initialized at the start of the program.
If the size of the WebAssembly memory would be constant, and indexing the
linear memory would wrap according to the memory size, things would work by
default. But neither of those is the case.
Perhaps the memory location is supposed to be initialized by some WebAssembly C
runtime library that needs to be linked to the program? Or is the WebAssembly
runtime environment expected to set that up somehow (seems wrong)?
I've managed to run LLVM-compiled WebAssembly programs (in a custom runtime) by
initializing the stack pointer with the size of the initial memory limit (and
thus assuming that the stack is located at the end of the initial linear
memory).
The attached test.c can be compiled to a valid WebAssembly program using
LLVM/clang compiled with the experimental WebAssembly target and the tools from
https://github.com/WebAssembly/binaryen and
https://github.com/WebAssembly/wabt:
$ clang --target=wasm32 -Os -S -o test.s test.c
$ s2wasm --start=main test.s > test.wast
$ wast2wasm -o test.wasm test.wast
I located the stack management code in
lib/Target/WebAssembly/WebAssemblyFrameLowering.cpp, but could't find a place
which attempts to initialize the __stack_pointer.
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