[BOLT] CDSplit Main Logic Part 1/3 #73082
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Note: this PR is 6/8 for upstreaming a new 3-way (hot-warm-cold) function splitting algorithm CDSplit. |
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.
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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.