| Issue |
130632
|
| Summary |
[llvm][opt][regression] `memcpyopt` pass inserts incorrect `memmove` call
|
| Labels |
|
| Assignees |
|
| Reporter |
HertzDevil
|
The following LLVM IR, which is already partially optimized from earlier passes, allocates two `{ i64, i32, i64 }`s and swaps their contents using a non-default allocator function: (for less minimal reproductions see https://github.com/crystal-lang/crystal/issues/15543)
```llvm
target datalayout = "e-m:w-p270:32:32-p271:32:32-p272:64:64-i64:64-i128:128-f80:128-n8:16:32:64-S128"
define void @main() {
alloca:
%0 = call ptr @GC_malloc(i64 8)
%1 = call ptr @GC_malloc(i64 48)
store ptr %1, ptr %0, align 8
%2 = getelementptr inbounds nuw i8, ptr %1, i64 24
call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %1, i8 18, i64 24, i1 false)
call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %2, i8 52, i64 24, i1 false)
%3 = load ptr, ptr %0, align 8
%4 = load i64, ptr %3, align 8
%5 = getelementptr inbounds nuw i8, ptr %3, i64 8
%6 = load { i32, i64 }, ptr %5, align 8
%7 = getelementptr inbounds nuw i8, ptr %1, i64 32
%8 = load { i32, i64 }, ptr %7, align 8
store i64 3761688987579986996, ptr %1, align 8
%9 = getelementptr inbounds nuw i8, ptr %1, i64 8
store { i32, i64 } %8, ptr %9, align 8
store i64 %4, ptr %2, align 8
store { i32, i64 } %6, ptr %7, align 8
ret void
}
declare ptr @GC_malloc(i64)
declare void @llvm.memset.p0.i64(ptr, i8, i64, i1)
```
Running just the `memcpyopt` pass used to produce the following:
```llvm
%0 = call ptr @GC_malloc(i64 8)
%1 = call ptr @GC_malloc(i64 48)
store ptr %1, ptr %0, align 8
%2 = getelementptr inbounds nuw i8, ptr %1, i64 24
call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %1, i8 18, i64 24, i1 false)
call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %2, i8 52, i64 24, i1 false)
%3 = load ptr, ptr %0, align 8
%4 = load i64, ptr %3, align 8
%5 = getelementptr inbounds nuw i8, ptr %3, i64 8
%6 = load { i32, i64 }, ptr %5, align 8
%7 = getelementptr inbounds nuw i8, ptr %1, i64 32
store i64 3761688987579986996, ptr %1, align 8
%8 = getelementptr inbounds nuw i8, ptr %1, i64 8
call void @llvm.memcpy.p0.p0.i64(ptr align 8 %8, ptr align 8 %7, i64 16, i1 false)
store i64 %4, ptr %2, align 8
store { i32, i64 } %6, ptr %7, align 8
ret void
```
Since LLVM 20 this becomes:
```llvm
%0 = call ptr @GC_malloc(i64 8)
%1 = call ptr @GC_malloc(i64 48)
store ptr %1, ptr %0, align 8
%2 = getelementptr inbounds nuw i8, ptr %1, i64 24
call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %1, i8 18, i64 24, i1 false)
call void @llvm.memset.p0.i64(ptr noundef nonnull align 1 dereferenceable(24) %2, i8 52, i64 24, i1 false)
%3 = load ptr, ptr %0, align 8
%4 = load i64, ptr %3, align 8
%5 = getelementptr inbounds nuw i8, ptr %3, i64 8
%6 = getelementptr inbounds nuw i8, ptr %1, i64 32
store i64 3761688987579986996, ptr %1, align 8
%7 = getelementptr inbounds nuw i8, ptr %1, i64 8
call void @llvm.memcpy.p0.p0.i64(ptr align 8 %7, ptr align 8 %6, i64 16, i1 false)
call void @llvm.memmove.p0.p0.i64(ptr align 8 %6, ptr align 8 %5, i64 16, i1 false)
store i64 %4, ptr %2, align 8
ret void
```
The new call to `llvm.memmove.p0.p0.i64` seems to assume that `%5` and `%7` can never alias, when they should be identical here. Adding `noalias` to `GC_malloc` does not appear to help, and I doubt this attribute is necessary.
The first PR that broke this was #108535.
[Compiler Explorer link](https://godbolt.org/z/Gfeja4M6c)
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