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Differential D78416

[docs][NFC] Mention instruction selectors in LLVM
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Authored by xgupta on Apr 17 2020, 10:03 PM.

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rnk
gribozavr
Summary

Mention Instruction Selectors in LLVM.

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xgupta created this revision.Apr 17 2020, 10:03 PM
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Harbormaster failed remote builds in B53823: Diff 258477!Apr 17 2020, 10:20 PM
craig.topper added a subscriber: craig.topper.Apr 18 2020, 12:00 AM
craig.topper added inline comments.
llvm/docs/CodeGenerator.rst
725

This doesn't mention -O0. It's also not clear to me why what happens for X86 is relevant in this section since it appears to be target independent.

xgupta added a comment.Apr 18 2020, 2:47 AM

Thank you sir for reviewing. FastIsel is also enabled at all optimization level for X86 & Sparc(I tested) because every time I need to pass -fast-isel=false for viewing dags using llc. I didn't built LLVM to try other targets.
I think FastIsel is default for all target? I will check & update X86 to all if it is.

xgupta updated this revision to Diff 258504.Apr 18 2020, 3:09 AM
xgupta retitled this revision from [docs][NFC] Mention FastISel as a default instruction selector for X86 target to [docs][NFC] Mention FastISel as a default instruction selector..
xgupta edited the summary of this revision. (Show Details)
xgupta removed a subscriber: craig.topper.

Update Title & Summary

xgupta added a subscriber: craig.topper.Apr 18 2020, 3:11 AM
Harbormaster failed remote builds in B53837: Diff 258504!Apr 18 2020, 3:13 AM
craig.topper added a comment.Apr 18 2020, 10:34 AM
In D78416#1990386, @xgupta wrote:

Thank you sir for reviewing. FastIsel is also enabled at all optimization level for X86 & Sparc(I tested) because every time I need to pass -fast-isel=false for viewing dags using llc. I didn't built LLVM to try other targets.
I think FastIsel is default for all target? I will check & update X86 to all if it is.

Fastisel should only be the default if you pass -O0 to llc or the function has the optnone attribute.

xgupta updated this revision to Diff 258534.Apr 18 2020, 11:24 AM

Remove X86 section

xgupta added a comment.Apr 18 2020, 11:25 AM

Fastisel should only be the default if you pass -O0 to llc or the function has the optnone attribute.

sorry sir I forget to remove -fast-isel=false at other optimization level. I think I should remove the "FastIsel is the default instruction selector for X86 target" line from here.

Harbormaster failed remote builds in B53856: Diff 258534!Apr 18 2020, 11:50 AM
xgupta updated this revision to Diff 258536.Apr 18 2020, 11:50 AM
xgupta retitled this revision from [docs][NFC] Mention FastISel as a default instruction selector. to [docs][NFC] Mention instruction selectors in LLVM.
xgupta edited the summary of this revision. (Show Details)

Update Title & Summary

Harbormaster failed remote builds in B53858: Diff 258536!Apr 18 2020, 11:50 AM
xgupta updated this revision to Diff 258581.Apr 18 2020, 11:00 PM

Add a link of GlobalIsel webpage

Harbormaster failed remote builds in B53886: Diff 258581!Apr 18 2020, 11:09 PM
xgupta added a comment.EditedApr 24 2020, 1:43 PM

Changes are not important so closing request.

xgupta abandoned this revision.Apr 24 2020, 1:43 PM

Revision Contents

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llvm/
docs/
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Diff 258581

llvm/docs/CodeGenerator.rst

Show First 20 Lines • Show All 713 Lines • ▼ Show 20 Lines
.. _Instruction Selection: .. _Instruction Selection:
.. _instruction selection section: .. _instruction selection section:
Instruction Selection Instruction Selection
--------------------- ---------------------
Instruction Selection is the process of translating LLVM code presented to the Instruction Selection is the process of translating LLVM code presented to the
code generator into target-specific machine instructions. There are several code generator into target-specific machine instructions. There are several
well-known ways to do this in the literature. LLVM uses a SelectionDAG based well-known ways to do this in the literature. LLVM have three types of
instruction selector. instruction selector. 1. SelectionDAG 2. FastIsel 3. GlobalIsel.
craig.topperUnsubmitted
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This doesn't mention -O0. It's also not clear to me why what happens for X86 is relevant in this section since it appears to be target independent.

craig.topper: This doesn't mention -O0. It's also not clear to me why what happens for X86 is relevant in…
`GlobalIsel <https://llvm.org/docs/GlobalISel/index.html>`_ is intended to
be a replacement of SelectionDAG and FastIsel.
Portions of the DAG instruction selector are generated from the target Portions of the DAG instruction selector are generated from the target
description (``*.td``) files. Our goal is for the entire instruction selector description (``*.td``) files. Our goal is for the entire instruction selector
to be generated from these ``.td`` files, though currently there are still to be generated from these ``.td`` files, though currently there are still
things that require custom C++ code. things that require custom C++ code.
.. _SelectionDAG: .. _SelectionDAG:
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