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[BOLT] CDSplit Main Logic Part 2/3 #73083

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ShatianWang
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The second diff in a series of 3 that implements the main logic of
CDSplit. When the function order is [... X ... BF ... Y ...], a main
benefit of splitting the hot fragment of BF further into a hot and a
warm fragment is that function calls in the form of X->Y or Y->X will
become shorter (i.e., SrcBB and DstBB will become closer to each other)
as long as the new hot fragment of BF is smaller in size compared to
the original hot fragment. This diff implements a function that finds
all such "shortenable" calls in the form of X->Y or Y->X for the given
function BF.

@ShatianWang ShatianWang marked this pull request as ready for review November 22, 2023 04:41
@ShatianWang
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Note: this PR is 7/8 for upstreaming a new 3-way (hot-warm-cold) function splitting algorithm CDSplit.
It is one commit ahead of #73082

This commit modifies BinaryContext::calculateEmittedSize to update the
BinaryBasicBlock::OutputAddressRange for each basic block in the input
BF. The modification is done in place, where BB.OutputAddressRange.second
less BB.OutputAddressRange.first now gives the emitted size of the basic
block.
This commit updates SplitFunctions.h and SplitFunctions.cpp to enable
the reuse of createEHTrampolines, mergeEHTrampolines, hasFullProfile,
and allBlocksCold by a distinct function splitting pass (CDSplit).
This commit establishes the general structure of the CDSplit
implementation without incorporating the exact splitting logic.
Currently, all functions undergo hot-cold splitting based on the
decisions made by the SplitFunctions pass. Subsequent commits
will introduce the precise splitting logic.
This commit explicitly adds a warm code section, .text.warm, when the
-use-cdsplit=1 flag is set. This replaces the previous approach of using
.text.cold.0 as warm and .text.cold.1 as cold in 3-way splitting.
This diff defines and initializes auxiliary variables used by CDSplit.
The first diff in a series of 3 that implements the main logic of
CDSplit. Under X86, function splitting can lead to block size increase.
This is because conditional and unconditional branch instructions whose
offset is under 8 bits can be encoded with 2 bytes. If the offset is
greater than 8 bits, then they need 6 and 5 bytes respectively.
Splitting a short conditional / unconditional branch will thus increase
the size of the src basic block by 4 and 3 bytes respectively. CDSplit
takes into account the potential block size increase when it makes
splitting decisions. This diff implements a function
estimatePostSplitBBAddress in CDSplit that approximates the block level
size increase at the given split index of the given function.
The second diff in a series of 3 that implements the main logic of
CDSplit. When the function order is [... X ... BF ... Y ...], a main
benefit of splitting the hot fragment of BF further into a hot and a
warm fragment is that function calls in the form of X->Y or Y->X will
become shorter (i.e., SrcBB and DstBB will become closer to each other)
as long as the new hot fragment of BF is smaller in size compared to
the original hot fragment. This diff implements a function that finds
all such "shortenable" calls in the form of X->Y or Y->X for the given
function BF.
@ShatianWang ShatianWang force-pushed the 112123Commit7_CDSplitMainLogic2 branch from 3b10e11 to 5d0647f Compare November 22, 2023 17:12
@ShatianWang ShatianWang marked this pull request as draft November 23, 2023 20:41
@ShatianWang ShatianWang deleted the 112123Commit7_CDSplitMainLogic2 branch December 21, 2023 05:36
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