[llvm-dev] In ISel, where Constant<0> comes from? (original) (raw)
Gleb Popov via llvm-dev llvm-dev at lists.llvm.org
Mon Dec 17 06:08:30 PST 2018
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Hello, LLVM devs.
I'm compiling the following simple IR:
define dso_local i32 @main(i32 %argc, i8** %argv) { entry: %retval = alloca i32, align 4 %argc.addr = alloca i32, align 4 %argv.addr = alloca i8**, align 8 store i32 0, i32* %retval, align 4 store i32 %argc, i32* %argc.addr, align 4 store i8** %argv, i8*** %argv.addr, align 8 ret i32 0 }
using llc -march=sparc -debug-only=isel.
In the Initial selection DAG I see
t19: ch,glue = CopyToReg t17, Register:i32 $i0, Constant:i32<0>
line. The same "Constant:i32<0>" node I see in my toy backend, which forces me to add a pattern that lowers it using "xor reg,reg". Much like "or g0,g0" pattern in SPARC.
However, I don't see that Constant node when compiling using X86 backend. How does it achieve this? And why initial DAGs are different at all? I got impression that initial DAG is fully target-independent, so these DAG should be the same before starting ISel. Am I wrong?
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