[llvm-dev] How to get the case value from Machine Instruction

[Martin J. O'Riordan via llvm-dev]

Some glitch in the emailer? I have received this message 3 times in a row!?

 

I think that by the time it gets as far as MI-level there is no reversible
method of determining the 'case' label at all.  The reason I say this, is
that I have often seen optimisations that coalesce groups of values into
interesting logical tests and jump-tables are completely avoided.  For
example, a simple range like:

 

switch(n) {

case 8:

case 9:

case 10:

case 11:

case 12:

case 13:

case 14:

case 15:

...action...

}

 

is quite likely to become (equivalent to):

 

if ((n & 0xFFFFFF8) == 0x00000008) ...action...

 

so the 'case' logic of the original expression is lost; and there are other
logical reductions a lot less obvious than this.  I expect that at IR-level,
especially pre-SCEV that the 'case' specific information might still be
available but at MI-level it has already been lost.  The debug information
might still retain some back-association, but I am not particularly familiar
with Dwarf.

 

            MartinO

 

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of yao via
llvm-dev
Sent: 09 April 2018 09:30
To: llvmdev <llvmdev at cs.uiuc.edu>
Subject: [llvm-dev] How to get the case value from Machine Instruction

 

Hi, guys

I am interesting about how to get the switch case value form the Machine
Instruction.

 

I know the switch will be converted to jump-table in the Machine
Instruction.

And in the phase CodeGen ,  the case-value of SwitchInst can get esasly.

but it seems no case -value in Machine Instruction.


The MI as follows:
Frame Objects:

  fi#0: size=1, align=0, at location [SP]

  fi#1: size=4, align=4, at location [SP+8]

  fi#2: size=4, align=4, at location [SP+4]

  fi#3: size=4, align=4, at location [SP]

Jump Tables:

%jump-table.0:  %bb.2 %bb.3 %bb.4 %bb.5

 

%bb.0: derived from LLVM BB %0

%r0 = MOVi 0, 14, %noreg, %noreg

STRi12 %r0, %stack.1, 14, %noreg

%r0 = MOVi 4, 14, %noreg, %noreg

STRi12 %r0, %stack.2, 14, %noreg

%r0 = LDRi12 %stack.2, 14, %noreg

%r0 = SUBri %r0, 1, 14, %noreg, %noreg

CMPri %r0, 3, 14, %noreg, implicit-def %cpsr

STRi12 %r0, %stack.3, 14, %noreg

Bcc %bb.6, 8, %cpsr

    Successors according to CFG: %bb.6 %bb.1

 

%bb.1: derived from LLVM BB %1

    Predecessors according to CFG: %bb.0

%1:gprnopc = LEApcrelJT %jump-table.0, 14, %noreg

%2:gprnopc = LDRrs killed %0:gprnopc, %1, 0, 14, %noreg; mem:LD4[JumpTable]

BR_JTr killed %2, %jump-table.0

    Successors according to CFG: %bb.2(?%) %bb.3(?%) %bb.4(?%) %bb.5(?%)

 

%bb.2: derived from LLVM BB %2

    Predecessors according to CFG: %bb.1

%r0 = LDRi12 %stack.2, 14, %noreg

%r0 = ADDri %r0, 11, 14, %noreg, %noreg

STRi12 %r0, %stack.2, 14, %noreg

B %bb.6

    Successors according to CFG: %bb.6

 

%bb.3: derived from LLVM BB %3

    Predecessors according to CFG: %bb.1

%r0 = LDRi12 %stack.2, 14, %noreg

%r0 = ADDri %r0, 12, 14, %noreg, %noreg

STRi12 %r0, %stack.2, 14, %noreg

B %bb.6

    Successors according to CFG: %bb.6

 

%bb.4: derived from LLVM BB %4

    Predecessors according to CFG: %bb.1

%r0 = LDRi12 %stack.2, 14, %noreg

%r0 = ADDri %r0, 13, 14, %noreg, %noreg

STRi12 %r0, %stack.2, 14, %noreg

B %bb.6

    Successors according to CFG: %bb.6

 

%bb.5: derived from LLVM BB %5

    Predecessors according to CFG: %bb.1

%r0 = LDRi12 %stack.2, 14, %noreg

%r0 = ADDri %r0, 14, 14, %noreg, %noreg

STRi12 %r0, %stack.2, 14, %noreg

    Successors according to CFG: %bb.6

 

%bb.6: derived from LLVM BB %6

    Predecessors according to CFG: %bb.0 %bb.2 %bb.3 %bb.4 %bb.5

%r0 = LDRi12 %stack.2, 14, %noreg

BX_RET 14, %noreg

 

I  hope to get the Case value from the MI, 
Thanks,
yaoxiao

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