Gleb Popov via llvm-dev
2018-Dec-17 14:08 UTC
[llvm-dev] In ISel, where Constant<0> comes from?
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? Thanks in advance. -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181217/d1b5f551/attachment.html>
Krzysztof Parzyszek via llvm-dev
2018-Dec-17 14:40 UTC
[llvm-dev] In ISel, where Constant<0> comes from?
On 12/17/2018 8:08 AM, Gleb Popov via llvm-dev wrote:> > 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?The selection DAG is very much target-specific. The differences in the initial DAG usually come from lowering function arguments and return values, and from lowering calls to other functions. This is where different calling conventions are applied, so the initial DAG may be different even for the same target if you change the calling convention. Later on more differences appear from legalization (which each target needs to customize to match its needs), and custom DAG combines. All of this happens before the actual selection process starts. -Krzysztof -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
Krzysztof Parzyszek via llvm-dev
2018-Dec-17 14:45 UTC
[llvm-dev] In ISel, where Constant<0> comes from?
On 12/17/2018 8:08 AM, Gleb Popov via llvm-dev wrote:> > In the Initial selection DAG I see > > t19: ch,glue = CopyToReg t17, Register:i32 $i0, Constant:i32<0>To answer this---this seems to be a part of the return sequence, i.e. the part of the calling convention that dictates how a function passes return values to its caller. This is handled via LowerReturn in the target lowering object. Check SparcTargetLowering::LowerReturn for the Sparc implementation. -Krzysztof -- Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, hosted by The Linux Foundation
Gleb Popov via llvm-dev
2018-Dec-18 07:10 UTC
[llvm-dev] In ISel, where Constant<0> comes from?
Thanks Tim and Krzysztof for pointing me in the right direction. Indeed, when I started my backend I just blindly copied LowerFormalArguments and LowerReturn from SPARC backend, and that's where these FrameIndex'es and Constant's are coming from. However, I haven't managed to get a "Constant<>" in the DAG when compiling for X86. I'm interested in how it is lowered. Can you please give me some guidance on this? -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20181218/626c5511/attachment.html>
Tim Northover via llvm-dev
2018-Dec-18 09:02 UTC
[llvm-dev] In ISel, where Constant<0> comes from?
On Tue, 18 Dec 2018 at 07:11, Gleb Popov via llvm-dev <llvm-dev at lists.llvm.org> wrote:> However, I haven't managed to get a "Constant<>" in the DAG when compiling for X86. I'm interested in how it is lowered. Can you please give me some guidance on this?How are you looking? When I run "llc -mtriple=x86_64-linux-gnu -debug-only=isel" on your IR I get multiple instances of Constants. At the very start is: Initial selection DAG: %bb.0 'main:entry' SelectionDAG has 18 nodes: t0: ch = EntryToken t7: i64 = Constant<0> t9: ch = store<(store 4 into %ir.retval)> t0, Constant:i32<0>, FrameIndex:i64<0>, undef:i64 t2: i32,ch = CopyFromReg t0, Register:i32 %0 t11: ch = store<(store 4 into %ir.argc.addr)> t9, t2, FrameIndex:i64<1>, undef:i64 t4: i64,ch = CopyFromReg t0, Register:i64 %1 t13: ch = store<(store 8 into %ir.argv.addr)> t11, t4, FrameIndex:i64<2>, undef:i64 t16: ch,glue = CopyToReg t13, Register:i32 $eax, Constant:i32<0> t17: ch = X86ISD::RET_FLAG t16, TargetConstant:i32<0>, Register:i32 $eax, t16:1 where the t16 line corresponds to what you've seen on SPARC. Cheers. Tim.
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