On Wednesday, 7 February 2018 at 13:50:06 UTC, Bastiaan Veelo
wrote:
On Wednesday, 7 February 2018 at 11:37:19 UTC, Michael wrote:
Does it work with slightly varied examples like where a = -1,
and is incremented etc.?
I played with it here https://run.dlang.io/is/15sr6c and every
variation I tried worked (correctly). So the example seems to
be pretty minimal. Maybe someone understanding assembly is able
to see what is going on after pressing that nice [ASM] button?
You don't need to understand assembler to grasp what's going on.
A printf does a good job too:
printf("%d\n", a); // 1908874352
It's even more fun, reduce the size of the static array to 8:
ubyte[8][1] data; // works
However, with 7 it fails again:
ubyte[7][1] data;
If you look at the assembly you will see that the compiler didn't
even include the assert calls for even static arrays:
https://run.dlang.io/is/Qkt1nA (-O)
https://run.dlang.io/is/7UfmXJ (-O [8])
https://run.dlang.io/is/QrtNeI (normal)
Here's an annotated excerpt from the normal build:
---
dec qword ptr -010h[RBP]
xor EDX,EDX ; a--
mov -8[RBP],DL
test DL,DL ; if (b)
je L5C
jmp short L18 ; goto loop
L5C: cmp qword ptr -010h[RBP],0FFFFFFFFFFFFFFFFh
je L78 ; assert(a == -1)
mov DL,0Ah
lea RSI,_TMP0@PC32[RIP]
lea RDI,_TMP0@PC32[RIP]
call __assert@PLT32
L78: xor EAX,EAX
---
The same looks slightly different with -O.
Here with printf and a different value to be compared with a
because it's easier to read:
---
main:
push RBP
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
----
So the optimizer seems to be confused and wrongly precomputes a.
Note that if you change something, a will be correctly statically
set in the printf too:
---
mov RSI,0FFFFFFFFh ; look ma - a is now -1
lea RDI,FLAT:.rodata[00h][RIP]
xor EAX,EAX ; set eax to 0
call printf@PLT32
---
