[llvm-dev] Backend subtraction changed to negative addition

[Matt Arsenault via llvm-dev]

On 01/25/2017 01:05 PM, Philip Herron via llvm-dev wrote:
> Hi all,
>
> I am writing a custom backend. Doing more testing i notice that for 
> some reason something like:
>
> int test(int x) { return x - 1; }
>
> is being turned into this IR:
>
> ; Function Attrs: nounwind
> define i32 @test(i32 %n) #0 {
> entry:
>   %n.addr = alloca i32, align 4
>   store i32 %n, i32* %n.addr, align 4
>   %0 = load i32* %n.addr, align 4
>   %sub = sub nsw i32 %0, 1
>   ret i32 %sub
> }
>
> But finally in code generation i am getting:
>
> ldc r2, #-1
> add r0, r2, r0
>
> Should this not be doing:
>
> ldc r2 #1
> sub r0 r2 r0
>
> I have defined both my add and sub instructions:
>
> def ADD : ALUInst<0b0001, (outs GRRegs:$dst),
>                    (ins GRRegs:$src1, GRRegs:$src2),
>                    "add $src1, $src2, $dst",
>                    [(set i32:$dst, (add i32:$src1, i32:$src2))]>;
>
> def SUB : ALUInst<0b0010, (outs GRRegs:$dst),
>                    (ins GRRegs:$src1, GRRegs:$src2),
>                    "sub $src1, $src2, $dst",
>                    [(set i32:$dst, (sub i32:$src1, i32:$src2))]>;
>
> Is there a way to override this behaviour?

You can add an optimization pattern for this. I have a patch to do this 
for AMDGPU since it costs code size in some cases as an example: 
https://reviews.llvm.org/D28043

-Matt