forked from OSchip/llvm-project
6398903ac8
We have the `clang -cc1` command-line option `-funwind-tables=1|2` and
the codegen option `VALUE_CODEGENOPT(UnwindTables, 2, 0) ///< Unwind
tables (1) or asynchronous unwind tables (2)`. However, this is
encoded in LLVM IR by the presence or the absence of the `uwtable`
attribute, i.e. we lose the information whether to generate want just
some unwind tables or asynchronous unwind tables.
Asynchronous unwind tables take more space in the runtime image, I'd
estimate something like 80-90% more, as the difference is adding
roughly the same number of CFI directives as for prologues, only a bit
simpler (e.g. `.cfi_offset reg, off` vs. `.cfi_restore reg`). Or even
more, if you consider tail duplication of epilogue blocks.
Asynchronous unwind tables could also restrict code generation to
having only a finite number of frame pointer adjustments (an example
of *not* having a finite number of `SP` adjustments is on AArch64 when
untagging the stack (MTE) in some cases the compiler can modify `SP`
in a loop).
Having the CFI precise up to an instruction generally also means one
cannot bundle together CFI instructions once the prologue is done,
they need to be interspersed with ordinary instructions, which means
extra `DW_CFA_advance_loc` commands, further increasing the unwind
tables size.
That is to say, async unwind tables impose a non-negligible overhead,
yet for the most common use cases (like C++ exceptions), they are not
even needed.
This patch extends the `uwtable` attribute with an optional
value:
- `uwtable` (default to `async`)
- `uwtable(sync)`, synchronous unwind tables
- `uwtable(async)`, asynchronous (instruction precise) unwind tables
Reviewed By: MaskRay
Differential Revision: https://reviews.llvm.org/D114543
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| llvm | ||
| README.txt | ||
| build.sh | ||
| conftest.go | ||
README.txt
This directory contains LLVM bindings for the Go programming language
(http://golang.org).
Prerequisites
-------------
* Go 1.2+.
* CMake (to build LLVM).
Using the bindings
------------------
The package path "llvm.org/llvm/bindings/go/llvm" can be used to
import the latest development version of LLVM from SVN. Paths such as
"llvm.org/llvm.v36/bindings/go/llvm" refer to released versions of LLVM.
It is recommended to use the "-d" flag with "go get" to download the
package or a dependency, as an additional step is required to build LLVM
(see "Building LLVM" below).
Building LLVM
-------------
The script "build.sh" in this directory can be used to build LLVM and prepare
it to be used by the bindings. If you receive an error message from "go build"
like this:
./analysis.go:4:84: fatal error: llvm-c/Analysis.h: No such file or directory
#include <llvm-c/Analysis.h> // If you are getting an error here read bindings/go/README.txt
or like this:
./llvm_dep.go:5: undefined: run_build_sh
it means that LLVM needs to be built or updated by running the script.
$ $GOPATH/src/llvm.org/llvm/bindings/go/build.sh
Any command line arguments supplied to the script are passed to LLVM's CMake
build system. A good set of arguments to use during development are:
$ $GOPATH/src/llvm.org/llvm/bindings/go/build.sh -DCMAKE_BUILD_TYPE=Debug -DLLVM_TARGETS_TO_BUILD=host -DBUILD_SHARED_LIBS=ON
Note that CMake keeps a cache of build settings so once you have built
LLVM there is no need to pass these arguments again after updating.
Alternatively, you can build LLVM yourself, but you must then set the
CGO_CPPFLAGS, CGO_CXXFLAGS and CGO_LDFLAGS environment variables:
$ export CGO_CPPFLAGS="`/path/to/llvm-build/bin/llvm-config --cppflags`"
$ export CGO_CXXFLAGS=-std=c++14
$ export CGO_LDFLAGS="`/path/to/llvm-build/bin/llvm-config --ldflags --libs --system-libs all`"
$ go build -tags byollvm
If you see a compilation error while compiling your code with Go 1.9.4 or later as follows,
go build llvm.org/llvm/bindings/go/llvm: invalid flag in #cgo LDFLAGS: -Wl,-headerpad_max_install_names
you need to setup $CGO_LDFLAGS_ALLOW to allow a compiler to specify some linker options:
$ export CGO_LDFLAGS_ALLOW='-Wl,(-search_paths_first|-headerpad_max_install_names)'