| Issue |
136353
|
| Summary |
[WebAssembly] Incorrect stackification of effectful instructions
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
SingleAccretion
|
Reading the stackification code, I came across this in `WebAssemblyRegStackify.cpp / static void query`:
```
// These instructions have hasUnmodeledSideEffects() returning true
// because they trap on overflow and invalid so they can't be arbitrarily
// moved, however in the specific case of register stackifying, it is safe
// to move them because overflow and invalid are Undefined Behavior.
```
Which prompted me to think about this this example:
```llvm
target datalayout = "e-m:e-p:32:32-p10:8:8-p20:8:8-i64:64-n32:64-S128-ni:1:10:20"
target triple = "wasm32-unknown-wasi"
declare i32 @extern_func(i32, i32)
define i32 @func(i32 %0, i32 %1) {
%call_value = call i32 @extern_func(i32 %0, i32 %1)
%div = udiv i32 %0, %1
%sub = sub i32 %div, %call_value
ret i32 %sub
}
```
Running this through `llc` we get:
```
> llc test.ll -o test.o --filetype=obj -O1 && wasm-objdump -d test.o
test.o: file format wasm 0x1
Code Disassembly:
000055 func[1] <func>:
000056: 20 00 | local.get 0
000058: 20 01 | local.get 1
00005a: 6e | i32.div_u
00005b: 20 00 | local.get 0
00005d: 20 01 | local.get 1
00005f: 10 80 80 80 80 00 | call 0 <env.extern_func>
000065: 6b | i32.sub
000066: 0b | end
```
This `udiv` and the call get swapped. This does not look correct, since we now have _introduced UB_ (a trap) under the following conditions:
1) The divisor (`%1`) is zero.
2) `extern_func` throws (either a WASM exception or a JS exception).
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