[llvm-dev] Backend subtraction changed to negative addition

[Philip Herron via llvm-dev]

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?

Thanks

--Phil
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170125/85ca7ab4/attachment.html>