| Issue |
71440
|
| Summary |
Miscompile from AArch64 Global ISel backend: UDiv on LLVM>17
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tanmaytirpankar
|
Look at the function `f` below:
```llvm
target datalayout = "e-m:e-i8:8:32-i16:16:32-i64:64-i128:128-n32:64-S128"
target triple = "aarch64-linux-gnu"
define i2 @f(i2 %0) {
%2 = sext i1 true to i2
%3 = udiv i2 1, %2
ret i2 %3
}
```
For the invocation `f(-1)`, the expectation is that `f` will return a 0.
Lets look at the assembly generated by llc.
```assembly
tanmay@revenant:~/llvm-clone/llvm/build-release/bin$ llc --march=aarch64 foo.ll -o -
.text
.file "foo.ll"
.globl f // -- Begin function f
.p2align 2
.type f,@function
f: // @f
.cfi_startproc
// %bb.0:
mov w0, wzr
ret
.Lfunc_end0:
.size f, .Lfunc_end0-f
.cfi_endproc
// -- End function
.section ".note.GNU-stack","",@progbits
```
Look at `%bb.0`. It stores a 0 in `w0`. According to the [procedure call standard for AArch64](https://github.com/ARM-software/abi-aa/blob/2a70c42d62e9c3eb5887fa50b71257f20daca6f9/aapcs64/aapcs64.rst#general-purpose-registers) the function returns the value stored in `w0` which as expected is 0.
Let us look at the assembly after running `--global-isel`:
```assembly
tanmay@revenant:~/llvm-clone/llvm/build-release/bin$ llc --global-isel --march=aarch64 foo.ll -o -
.text
.file "foo.ll"
.globl f // -- Begin function f
.p2align 2
.type f,@function
f: // @f
.cfi_startproc
// %bb.0:
ret
.Lfunc_end0:
.size f, .Lfunc_end0-f
.cfi_endproc
// -- End function
.section ".note.GNU-stack","",@progbits
```
As can be seen here in `%bb.0`, there are no instructions. the function returns the value stored in `w0` which has the first argument stored in it so -1.
The behavior was correct up until llvm-17.
cc @regehr
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