[LLVMdev] Splitting live ranges of half-defined registers

[Krzysztof Parzyszek]

I have already written about something similar (either on the list, or 
in private communication), so this may look familiar.

Here's a scenario I'm observing:

First, we have some innocent looking code:
    vreg(32) = x                  // vreg(32) = 32-bit register
    ... = vreg(32)
    [...]
    vreg(64).low_half = vreg(32)  // vreg(64) = 64-bit register
    [...]

then, after register coalescing:
    vreg(64).low_half = x
    ... = vreg(64).low_half
    [...]
    [etc.]

then, the live range of the 64-bit vreg is split during register allocation:
    vreg'(64).low_half = x     // first live range starts
    ... = vreg'(64).low_half
    vreg"(64) = vreg'(64)      // next live range starts
    [...]

The problem is that vreg' only has half of it defined, and so the 
assignment vreg" = vreg' copies a register which has half of it undefined.


My question is: is this something that was a part of the design?


The problem may arise if someone, for whatever reason, decides to 
rewrite the 64-bit copy as two 32-bit copies, e.g.
     vreg"(32).low_half  = vreg'(32).low_half     (1)
     vreg"(32).high_half = vreg'(32).high_half    (2)
When this happens, the assignment (2) basically reads an undefined register.



A scenario like this happens in real life.  Look at vreg304 below:

BB#2: derived from LLVM BB %if.end
     Predecessors according to CFG: BB#1
         %vreg61<def> = LDrih_indexed %vreg56, 3134; IntRegs:%vreg61,%vreg56
         %vreg62<def> = LDriuh_indexed %vreg56, 680; IntRegs:%vreg62,%vreg56
         %R1<def> = TFRI 1431655766
         ADJCALLSTACKDOWN 0, %R29<imp-def>, %R30<imp-def>, 
%R31<imp-use>, %R30<imp-use>, %R29<imp-use>
         %vreg304:subreg_loreg<def,read-undef> = ADDri_SUBr_V4 %vreg62, 
5, %vreg61; DoubleRegs:%vreg304 IntRegs:%vreg62,%vreg61
         %vreg519<def> = TFRI 3148; IntRegs:%vreg519
         %vreg523<def> = TFRI64 15; DoubleRegs:%vreg523
         %R0<def> = COPY %vreg304:subreg_loreg; DoubleRegs:%vreg304
         CALLv3 <ga:@foo>, %D0<imp-def,dead>, %D1<imp-def,dead>, 
%D2<imp-def,dead>, %R31<imp-def>, %R0<imp-use>, %R1<imp-use>, 
%R0<imp-def>, ...


Then, after live range splitting, we have vreg539 and vreg537, and right 
before the call, a copy from half-undefined vreg539:

BB#2: derived from LLVM BB %if.end
     Predecessors according to CFG: BB#1
         %vreg61<def> = LDrih_indexed %vreg56, 3134; 
IntRegs:%vreg61,%vreg56
         %vreg62<def> = LDriuh_indexed %vreg56, 680; IntRegs:%vreg62,%vreg56
         %R1<def> = TFRI 1431655766
         ADJCALLSTACKDOWN 0, %R29<imp-def>, %R30<imp-def>, 
%R31<imp-use>, %R30<imp-use>, %R29<imp-use>
         %vreg539:subreg_loreg<def,read-undef> = ADDri_SUBr_V4 %vreg62, 
5, %vreg61; DoubleRegs:%vreg539 IntRegs:%vreg62,%vreg61
         %vreg519<def> = TFRI 3148; IntRegs:%vreg519
         %vreg523<def> = TFRI64 15; DoubleRegs:%vreg523
         %R0<def> = COPY %vreg539:subreg_loreg; DoubleRegs:%vreg539
         %vreg537<def> = COPY %vreg539; DoubleRegs:%vreg537,%vreg539
         CALLv3 <ga:@foo>, %D0<imp-def,dead>, %D1<imp-def,dead>, 
%D2<imp-def,dead>, %R31<imp-def>, %R0<imp-use>, %R1<imp-use>, 
%R0<imp-def>, ...


-Krzysztof

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