[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

--- Comment #8 from github-bugzi...@puremagic.com ---
Commits pushed to master at https://github.com/dlang/dmd

https://github.com/dlang/dmd/commit/27dd8373a06cafa7e6038bf853cf15ab9575518d
fix Issue 16189 - Optimizer bug, with simple test case

https://github.com/dlang/dmd/commit/88a963a8ce44d062d45c34677a697ff2bd1fe2fa
Merge pull request #8074 from WalterBright/fix16189

fix Issue 16189 - Optimizer bug, with simple test case

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[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

github-bugzi...@puremagic.com changed:

   What|Removed |Added

 Status|NEW |RESOLVED
 Resolution|--- |FIXED

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[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

--- Comment #7 from Walter Bright  ---
https://github.com/dlang/dmd/pull/8074

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[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

Walter Bright  changed:

   What|Removed |Added

   Hardware|x86_64  |All

--

[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

Walter Bright  changed:

   What|Removed |Added

   Keywords||industry
 CC||bugzi...@digitalmars.com

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[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

bitter.ta...@gmx.com changed:

   What|Removed |Added

 CC||bitter.ta...@gmx.com

--- Comment #6 from bitter.ta...@gmx.com ---
The LICM pass is too eager and given this small snippet
```
void main()
{
ubyte[9][1] data;
size_t a = 0;
loop:
data[0] = data[a];
a--;
bool b = false;
if (b) goto loop;
assert(a == -1); // Fails with -O
}
```
`a' is considered to be as an "index" variable into `data' with starting value
0 and that's decremented until `a < 1', this condition is most likely derived
by the bound-checking condition.
As a result we get

```
Adding (a(1) =  ((_TMP1(3) -  #data(0)) /  9LL )) to exit block B4
```

that doesn't play well with the constant folding pass

```
const prop (_TMP1(3) replaced by #data(0).xfff7)
```

as shown by

```
const prop (a(1) replaced by 2049638230412172400LL )
```

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[Issue 16189] Optimizer bug, with simple test case

[d-bugmail--- via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

Seb  changed:

   What|Removed |Added

 CC||greensunn...@gmail.com

--- Comment #5 from Seb  ---
See also the discussion in the forum:
https://forum.dlang.org/post/jguphfhffgtdtfitt...@forum.dlang.org

My summary:

The optimizer seems to be confused and wrongly precomputes a.
Note that if you remove e.g. "if (b) goto loop;", a will be correctly
statically set in the printf too:

---
mov RSI,0h; look ma - a is now -1
lea RDI,FLAT:.rodata[00h][RIP]
xor EAX,EAX   ; set eax to 0
call  printf@PLT32
---

For comparison the assembly of -O.
Here with printf and a different value to be compared with a because it's
easier to read:

---
main:
pushRBP
mov RBP,RSP
sub RSP,010h
lea RAX,-010h[RBP]
xor ECX,ECX
mov [RAX],RCX
mov 8[RAX],CL
lea RSI,-010h[RBP]
lea RDI,-010h[RBP]
movsd
movsb
mov RSI,01C71C71C71C71C70h  ; 2 function argument
(value of a)
lea RDI,FLAT:.rodata[00h][RIP]  ; "%d" (1st function
argument)
xor EAX,EAX ; set eax to 0
call  printf@PLT32  ; printf("%d", a)
mov EDX,0Ch
lea RSI,_TMP0@PC32[RIP] ; load function
arguments (in reverse order)
lea RDI,_TMP0@PC32[RIP]
call  __assert@PLT32; values load, let's
call assert
add [RAX],AL
.text.main  ends
end


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[Issue 16189] Optimizer bug, with simple test case

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

--- Comment #4 from Kirill Kryukov  ---
I thought this might be a regression, so I tested dmd versions down to 2.050.
But no, the bug is there in each and every one of them.

--

[Issue 16189] Optimizer bug, with simple test case

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

Ketmar Dark  changed:

   What|Removed |Added

 CC||ket...@ketmar.no-ip.org

--- Comment #3 from Ketmar Dark  ---
funny thing: adding `asm { nop; }` anywhere in `main` seems to turn the
optimizer off, and the code works again. a hackish way to control optimizer on
per-function basis. ;-)

--

[Issue 16189] Optimizer bug, with simple test case

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

--- Comment #2 from Kirill Kryukov  ---
A possible workaround: change "a--;" into "{ auto a2 = a - 1; a = a2; }".

(This is NOT to suggest that the bug does not need fixing, as it's annoying as
hell that even simplest C-like code does not work correctly.)

As for previous reduction - it hurts my eyes to see size_t (unsigned type)
compared for equality with -1, so I suggest to at least use ptrdiff_t.

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[Issue 16189] Optimizer bug, with simple test case

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

ag0ae...@gmail.com changed:

   What|Removed |Added

   Keywords||wrong-code
 CC||ag0ae...@gmail.com
 OS|Windows |All

--- Comment #1 from ag0ae...@gmail.com ---
Slightly more reduced:


void main()
{
ubyte[9][1] data;
size_t a = 0;
loop:
data[0] = data[a];
a--;
bool b = false;
if (b) goto loop;
assert(a == -1); // Fails with -O
}


Also happens on Linux.

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[Issue 16189] Optimizer bug, with simple test case

[via Digitalmars-d-bugs]

https://issues.dlang.org/show_bug.cgi?id=16189

Kirill Kryukov  changed:

   What|Removed |Added

 CC||kkryu...@gmail.com

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