Philip Herron via llvm-dev
2017-Jan-25 21:05 UTC
[llvm-dev] Backend subtraction changed to negative addition
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>
Matt Arsenault via llvm-dev
2017-Jan-25 21:23 UTC
[llvm-dev] Backend subtraction changed to negative addition
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
Philip Herron via llvm-dev
2017-Jan-26 00:43 UTC
[llvm-dev] Backend subtraction changed to negative addition
This looks to be exactly what i need. Though this instruction set i am working with does not support immedites. It has a load constant to register instruction is the only way to load immedites. So i am not sure if this pattern will pick that up correctly. Will give it a test. Thanks --Phil On 25 January 2017 at 21:23, Matt Arsenault <Matthew.Arsenault at amd.com> wrote:> 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 > >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170126/82569894/attachment.html>
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