[llvm-bugs] [Bug 43780] New: GVN removes necessary load iff store to the same address uses data-cast

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=43780

            Bug ID: 43780
           Summary: GVN removes necessary load iff store to the same
                    address uses data-cast
           Product: new-bugs
           Version: 9.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: kaz-marukawa at xr.jp.nec.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Following code is compiled wrongly with "-O3 -fno-unroll-loops" option.

```
#include <stdint.h>

#define SIZE 9
void test(float const* kern, float* output)
{
    float buf[SIZE];
    float* ptr = buf;
    extern void func(float*, float);

    for(int i = 0; i < 1; ++i){                 // loop_i
        for (int j = 0; j < SIZE; ++j){         // loop_j
            ((int32_t*)ptr)[0] = ((int32_t*)kern)[j];
            ptr += 1;
            // we have calculations here originally
        }
        // we have calculations here originally
    }
    func(output, buf[0]);
}
```

The point is 2nd argument of func function call.  LLVM generates undef
instead of buf[0] through optimizations.

To reproduce the problem, please save this test program as tp.c and does:
```
$ clang -O3 -fno-unroll-loops -S -emit-llvm tp.c
$ grep undef tp.ll
  tail call void @func(float* %output, float undef) #3
```

This test program is compiled fine for following cases, so this is not
a real problem, but it is better to be fixed...

- -O1 -fno-unroll-loops
- -O3 (without -fno-unroll-loops)
- Modify source code to copy data as float without cast.
- Modify source code to use single loop instead of doubly nested loops.

The debug message shows GVN is removing a load of buf[0] like below.  I

```
$ clang -O3 -fno-unroll-loops -S -emit-llvm tp.c -mllvm -debug
...
; we have load and call here
IC: Visiting:   %1 = load float, float* %arraydecay, align 16, !tbaa !2
IC: Visiting:   tail call void @func(float* %output, float %1) #3
...
; GVN removes the load by unknown reason
GVN REMOVING NONLOCAL LOAD:   %1 = load float, float* %arraydecay, align 16,
!tbaa !2
GVN removed:   %1 = load float, float* %arraydecay, align 16, !tbaa !2
...
; Now, %1 become undef.
  tail call void @func(float* %output, float undef) #3
```

I tried to investigate this issue by myself, but didn't find the reason of
this.
Hope someone help me or solve this issue.  Thanks.

In addition, I'm using fresh llvm 9.0.0.

```
$ clang --version
clang version 9.0.0 (git at ...:.../llvm-project.git
0399d5a9682b3cef71c653373e38890c63c4c365)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: ...
```

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