This patch teaches llvm-mca how to identify dependency breaking instructions on
btver2.
An example of dependency breaking instructions is the zero-idiom XOR (example:
`XOR %eax, %eax`), which always generates zero regardless of the actual value of
the input register operands.
Dependency breaking instructions don't have to wait on their input register
operands before executing. This is because the computation is not dependent on
the inputs.
Not all dependency breaking idioms are also zero-latency instructions. For
example, `CMPEQ %xmm1, %xmm1` is independent on
the value of XMM1, and it generates a vector of all-ones.
That instruction is not eliminated at register renaming stage, and its opcode is
issued to a pipeline for execution. So, the latency is not zero.
This patch adds a new method named isDependencyBreaking() to the MCInstrAnalysis
interface. That method takes as input an instruction (i.e. MCInst) and a
MCSubtargetInfo.
The default implementation of isDependencyBreaking() conservatively returns
false for all instructions. Targets may override the default behavior for
specific CPUs, and return a value which better matches the subtarget behavior.
In future, we should teach to Tablegen how to automatically generate the body of
isDependencyBreaking from scheduling predicate definitions. This would allow us
to expose the knowledge about dependency breaking instructions to the machine
schedulers (and, potentially, other codegen passes).
Differential Revision: https://reviews.llvm.org/D49310
llvm-svn: 338372
This is a short-term fix for PR38093.
For now, we llvm::report_fatal_error if the instruction builder finds an
unsupported instruction in the instruction stream.
We need to revisit this fix once we start addressing PR38101.
Essentially, we need a better framework for error handling.
llvm-svn: 336543
This simplifies the logic that updates RAW dependencies in the DispatchStage.
There is no advantage in storing that flag in the ReadDescriptor; we should
simply rely on the call to `STI.getReadAdvanceCycles()` to obtain the
ReadAdvance cycles. If there are no read-advance entries, then method
`getReadAdvanceCycles()` quickly returns 0.
No functional change intended.
llvm-svn: 335977
This patch teaches llvm-mca how to identify register writes that implicitly zero
the upper portion of a super-register.
On X86-64, a general purpose register is implemented in hardware as a 64-bit
register. Quoting the Intel 64 Software Developer's Manual: "an update to the
lower 32 bits of a 64 bit integer register is architecturally defined to zero
extend the upper 32 bits". Also, a write to an XMM register performed by an AVX
instruction implicitly zeroes the upper 128 bits of the aliasing YMM register.
This patch adds a new method named clearsSuperRegisters to the MCInstrAnalysis
interface to help identify instructions that implicitly clear the upper portion
of a super-register. The rest of the patch teaches llvm-mca how to use that new
method to obtain the information, and update the register dependencies
accordingly.
I compared the kernels from tests clear-super-register-1.s and
clear-super-register-2.s against the output from perf on btver2. Previously
there was a large discrepancy between the estimated IPC and the measured IPC.
Now the differences are mostly in the noise.
Differential Revision: https://reviews.llvm.org/D48225
llvm-svn: 335113
This patch is the last of a sequence of three patches related to LLVM-dev RFC
"MC support for variant scheduling classes".
http://lists.llvm.org/pipermail/llvm-dev/2018-May/123181.html
This fixes PR36672.
The main goal of this patch is to teach llvm-mca how to solve variant scheduling
classes. This patch does that, plus it adds new variant scheduling classes to
the BtVer2 scheduling model to identify so-called zero-idioms (i.e. so-called
dependency breaking instructions that are known to generate zero, and that are
optimized out in hardware at register renaming stage).
Without the BtVer2 change, this patch would not have had any meaningful tests.
This patch is effectively the union of two changes:
1) a change that teaches llvm-mca how to resolve variant scheduling classes.
2) a change to the BtVer2 scheduling model that allows us to special-case
packed XOR zero-idioms (this partially fixes PR36671).
Differential Revision: https://reviews.llvm.org/D47374
llvm-svn: 333909
This is required if we want to correctly match the behavior of method
SubtargetEmitter::ExpandProcResource() in Tablegen. When computing the set of
"consumed" processor resources and resource cycles, the logic in
ExpandProcResource() doesn't update the number of resource cycles contributed by
a "Super" resource to a group. We need to take this into account when a model
declares a processor resource which is part of a 'processor resource group', and
it is also used as the "Super" of other resources.
llvm-svn: 333892
The DEBUG() macro is very generic so it might clash with other projects.
The renaming was done as follows:
- git grep -l 'DEBUG' | xargs sed -i 's/\bDEBUG\s\?(/LLVM_DEBUG(/g'
- git diff -U0 master | ../clang/tools/clang-format/clang-format-diff.py -i -p1 -style LLVM
- Manual change to APInt
- Manually chage DOCS as regex doesn't match it.
In the transition period the DEBUG() macro is still present and aliased
to the LLVM_DEBUG() one.
Differential Revision: https://reviews.llvm.org/D43624
llvm-svn: 332240
Before, the instruction builder incorrectly assumed that only explicit reads
could have been associated with ReadAdvance entries.
This patch fixes the issue and adds a test to verify it.
llvm-svn: 328972
Summary:
r327219 added wrappers to std::sort which randomly shuffle the container before sorting.
This will help in uncovering non-determinism caused due to undefined sorting
order of objects having the same key.
To make use of that infrastructure we need to invoke llvm::sort instead of std::sort.
Note: This patch is one of a series of patches to replace *all* std::sort to llvm::sort.
Refer the comments section in D44363 for a list of all the required patches.
Reviewers: JDevlieghere, zturner, echristo, dberris, friss
Reviewed By: echristo
Subscribers: gbedwell, llvm-commits
Differential Revision: https://reviews.llvm.org/D45141
llvm-svn: 328943
The tool was passing the wrong operand index to method
MCSubtargetInfo::getReadAdvanceCycles(). That method requires a "UseIdx", and
not the operand index. This was found when testing X86 code where instructions
had a memory folded operand.
This patch fixes the issue and adds test read-advance-1.s to ensure that
the ReadAfterLd (a ReadAdvance of 3cy) information is correctly used.
llvm-svn: 328790
Function computeProcResourceMasks is used by the ResourceManager (owned by the
Scheduler) to compute resource masks for processor resources. Before this
refactoring, there was an implicit dependency between the Scheduler and the
InstrBuilder. That is because InstrBuilder has to know about resource masks when
computing the set of processor resources consumed by a new instruction.
With this patch, the functionality that computes resource masks has been
extracted from the ResourceManager, and moved to a separate file (Support.h).
This helps removing the dependency between the Scheduler and the InstrBuilder.
No functional change intended.
llvm-svn: 327973
Before this patch, the register file was always updated at instruction creation
time. That means, new read-after-write dependencies, and new temporary registers
were allocated at instruction creation time.
This patch refactors the code in InstrBuilder, and move all the logic that
updates the register file into the dispatch unit. We only want to update the
register file when instructions are effectively dispatched (not before).
This refactoring also helps removing a bad dependency between the InstrBuilder
and the DispatchUnit.
No functional change intended.
llvm-svn: 327514
llvm-mca is an LLVM based performance analysis tool that can be used to
statically measure the performance of code, and to help triage potential
problems with target scheduling models.
llvm-mca uses information which is already available in LLVM (e.g. scheduling
models) to statically measure the performance of machine code in a specific cpu.
Performance is measured in terms of throughput as well as processor resource
consumption. The tool currently works for processors with an out-of-order
backend, for which there is a scheduling model available in LLVM.
The main goal of this tool is not just to predict the performance of the code
when run on the target, but also help with diagnosing potential performance
issues.
Given an assembly code sequence, llvm-mca estimates the IPC (instructions per
cycle), as well as hardware resources pressure. The analysis and reporting style
were mostly inspired by the IACA tool from Intel.
This patch is related to the RFC on llvm-dev visible at this link:
http://lists.llvm.org/pipermail/llvm-dev/2018-March/121490.html
Differential Revision: https://reviews.llvm.org/D43951
llvm-svn: 326998
Andrea Di Biagio