[llvm-dev] some questions about DAG

[林政宗 via llvm-dev]

Hello.


I am reading the code about DAG to DAG Instruction selection when I meet some problems. I am confused about the using of ch, glue.

when will they be used? And what are they used for?

when a sdnode doesn't have an actual result, it would have an MVT::Other typed result. But how to decide which sdnode is the user of this result?
for example, I have the DAG before instruction selection step, the corresponding graph is in attachment:

SelectionDAG has 19 nodes:

t0: ch = EntryToken

t5: ch = store<ST4[%1]> t0, Constant:i32<11>, FrameIndex:i32<0>, undef:i32

t8: ch = store<Volatile ST4[%2]> t5, Constant:i32<12>, FrameIndex:i32<1>, undef:i32

t9: i32,ch = load<LD4[%1]> t8, FrameIndex:i32<0>, undef:i32

t12: i32,ch = load<Volatile LD4[%2]> t9:1, FrameIndex:i32<1>, undef:i32

t14: ch = store<ST4[%1]> t12:1, t22, FrameIndex:i32<0>, undef:i32

t17: ch,glue = CopyToReg t14, Register:i32 %V0, t22

t11: i32 = add t9, Constant:i32<1>

t20: glue = Cpu0ISD::DivRem t11, t12

t22: i32,ch,glue = CopyFromReg t0, Register:i32 %HI, t20

t18: ch = Cpu0ISD::Ret t17, Register:i32 %V0, t17:1

The .ll code is as below:

*********************************************************************************************************************************


; ModuleID = 'ch4_1_mod.bc'
source_filename = "ch4_1_mod.cpp"
target datalayout = "E-m:m-p:32:32-i8:8:32-i16:16:32-i64:64-n32-S64"
target triple = "mips-unknown-linux-gnu"

; Function Attrs: nounwind
define i32 @_Z8test_modv() #0 {
%1 = alloca i32, align 4
%2 = alloca i32, align 4
store i32 11, i32* %1, align 4
store volatile i32 12, i32* %2, align 4
%3 = load i32, i32* %1, align 4
%4 = add nsw i32 %3, 1
%5 = load volatile i32, i32* %2, align 4
%6 = srem i32 %4, %5
store i32 %6, i32* %1, align 4
%7 = load i32, i32* %1, align 4
ret i32 %7
}

attributes #0 = { nounwind "disable-tail-calls"="false" "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-frame-pointer-elim-non-leaf" "no-infs-fp-math"="false" "no-jump-tables"="false" "no-nans-fp-math"="false" "no-signed-zeros-fp-math"="false" "stack-protector-buffer-size"="8" "target-cpu"="mips32r2" "target-features"="+mips32r2" "unsafe-fp-math"="false" "use-soft-float"="false" }

!llvm.ident = !{!0}

!0 = !{!"clang version 3.9.0 (tags/RELEASE_390/final)"}


*********************************************************************************************************************************

Considering t9 already has a i32 sdvalue, why ch is need? Does ch represent for MVT::Other typed sdvalue?

t22 has 3 results. ch is one of them, but ch is not used by any node. why is it created?

what about glue? what is it used for and when is it used?

And when the DAG is processed, there is usually a variable chain. what is it used for? and how is it used? Does ch in DAG represent for chain?

Thanks a lot!













Jack





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