[LLVMdev] Hit a snag while attempting to write a backend - any advice?

Lee Hammerton savoury.snax at googlemail.com
Mon Mar 18 11:34:19 PDT 2013


 I've been experimenting with writing a backend for LLVM (3.2) (having
already written a frontend http://savourysnax.github.com/EDL), everything
was going reasonably ok ( calls/returns, epilogue, prologue, etc are all
working), up until I tried to place support for conditional  branches.

Given this simple program :

int test(int c,int d)
        if (c)
                return d;
                return c;

with optimisations disabled (opts enabled it generates a select and this
works fine) - I get the error  at the bottom of the post.

 It seems to state, the problem is in the usage of -  %R0 in : JMP <BB#3>,

what is confusing me, is the implied R0 on the jump, it only appears at
Post SSA, and i`m not sure where to look to discern what i've done to cause
it. I've tested the same code with the x86 backend and it never seems to do
this implied usage. I realise I've not left you a lot to go on, but I`m
really just looking for likely areas to go poke around in. The backend was
started by copying the MSP430 backend then commenting out large swathes of
things (I may have left something in that causes the below error). I'm not
using (or shouldn't be) any custom lowering, all instructions map to DAG
concepts (with the exception of select_cc and br_cc which are set to

Any pointers would be much appreciated - I can post code etc, if required.

 Thanks, Lee

# Machine code for function test: Post SSA
Frame Objects:
  fi#0: size=4, align=4, at location [SP]
  fi#1: size=4, align=4, at location [SP]
  fi#2: size=4, align=4, at location [SP]
Function Live Ins: %R0 in %vreg0, %R1 in %vreg1
Function Live Outs: %R0

BB#0: derived from LLVM BB %entry
    Live Ins: %R0 %R1
        %vreg1<def> = COPY %R1<kill>; GR32:%vreg1
        %vreg0<def> = COPY %R0; GR32:%vreg0
        MOV32mr <fi#1>, 0, %vreg0<kill>; mem:ST4[%c.addr] GR32:%vreg0
        MOV32mr <fi#2>, 0, %vreg1<kill>; mem:ST4[%d.addr] GR32:%vreg1
        %vreg2<def> = MOV32rm <fi#1>, 0; mem:LD4[%c.addr] GR32:%vreg2
        %vreg3<def> = CMPfri %vreg2<kill>, 0; SR1:%vreg3 GR32:%vreg2
        JCC %vreg3<kill>, <BB#2>; SR1:%vreg3
        JMP <BB#1>, %R0<imp-use>
    Successors according to CFG: BB#1(20) BB#2(12)

BB#1: derived from LLVM BB %if.then
    Predecessors according to CFG: BB#0
        %vreg5<def> = MOV32rm <fi#2>, 0; mem:LD4[%d.addr] GR32:%vreg5
        MOV32mr <fi#0>, 0, %vreg5<kill>; mem:ST4[%retval] GR32:%vreg5
        JMP <BB#3>, %R0<imp-use>
    Successors according to CFG: BB#3

BB#2: derived from LLVM BB %if.else
    Predecessors according to CFG: BB#0
        %vreg4<def> = MOV32rm <fi#1>, 0; mem:LD4[%c.addr] GR32:%vreg4
        MOV32mr <fi#0>, 0, %vreg4<kill>; mem:ST4[%retval] GR32:%vreg4
    Successors according to CFG: BB#3

BB#3: derived from LLVM BB %return
    Predecessors according to CFG: BB#2 BB#1
        %vreg6<def> = MOV32rm <fi#0>, 0; mem:LD4[%retval] GR32:%vreg6
        %R0<def> = COPY %vreg6<kill>; GR32:%vreg6
        RET %R0<imp-use,kill>

# End machine code for function test.

*** Bad machine code: Using an undefined physical register ***
- function:    test
- basic block: BB#1 if.then (0xea46f84)
- instruction: JMP <BB#3>, %R0<imp-use>
- operand 1:   %R0<imp-use>
LLVM ERROR: Found 1 machine code errors.
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