[lld] r288314 - Add `isRelExprOneOf` helper
Rafael Avila de Espindola via llvm-commits
llvm-commits at lists.llvm.org
Tue Dec 6 14:11:12 PST 2016
Thanks for the patch.
I uploaded a new version of the benchmark to
https://drive.google.com/open?id=0B7iRtublysV6RjFBcVJhSjg1eHM
The problem you had with it using files from my machine should be
fixed. The issue was with linker scripts using absolute paths. It can be
avoided with fakeroot, but I just edited the scripts to make it easier.
I got a smaller benefit from this patch. The results were
firefox-O0
master 4.150496547
patch 4.144422417 1.00146561556x faster
firefox
master 6.901552864
patch 6.900340356 1.00017571713x faster
firefox-gc
master 7.078655427
patch 7.069354855 1.00131561821x faster
chromium
master 4.107906591
patch 4.094586237 1.0032531624x faster
chromium fast
master 2.007441161
patch 1.999441702 1.00400084633x faster
the gold plugin
master 0.337859787
patch 0.334852395 1.0089812468x faster
clang
master 0.57712437
patch 0.574027448 1.00539507651x faster
llvm-as
master 0.033570448
patch 0.033285747 1.00855324052x faster
the gold plugin fsds
master 0.363855963
patch 0.36106724 1.00772355587x faster
clang fsds
master 0.656084922
patch 0.652941395 1.00481440911x faster
llvm-as fsds
master 0.033544621
patch 0.033198883 1.01041414556x faster
scylla
master 3.112523401
patch 3.103663875 1.00285453785x faster
Sean Silva via llvm-commits <llvm-commits at lists.llvm.org> writes:
> Author: silvas
> Date: Wed Nov 30 23:43:48 2016
> New Revision: 288314
>
> URL: http://llvm.org/viewvc/llvm-project?rev=288314&view=rev
> Log:
> Add `isRelExprOneOf` helper
>
> In various places in LLD's hot loops, we have expressions of the form
> "E == R_FOO || E == R_BAR || ..." (E is a RelExpr).
>
> Some of these expressions are quite long, and even though they usually go just
> a very small number of ways and so should be well predicted, they can still
> occupy branch predictor resources harming other parts of the code, or they
> won't be predicted well if they overflow branch predictor resources or if the
> branches are too dense and the branch predictor can't track them all (the
> compiler can in theory avoid this, at a cost in text size). And some of these
> expressions are so large and executed so frequently that even when
> well-predicted they probably still have a nontrivial cost.
>
> This speedup should be pretty portable. The cost of these simple bit tests is
> independent of:
>
> - the target we are linking for
> - the distribution of RelExpr's for a given link (which can depend on how the
> input files were compiled)
> - what compiler was used to compile LLD (it is just a simple bit test;
> hopefully the compiler gets it right!)
> - adding new target-dependent relocations (e.g. needsPlt doesn't pay any extra
> cost checking R_PPC_PLT_OPD on x86-64 builds)
>
> I did some rough measurements on clang-fsds and this patch gives over about 4%
> speedup for a regular -O1 link, about 2.5% for -O3 --gc-sections and over 5%
> for -O0. Sorry, I don't have my current machine set up for doing really
> accurate measurements right now.
>
> This also is just a bit cleaner. Thanks for Joerg for suggesting for
> this approach.
>
> Differential Revision: https://reviews.llvm.org/D27156
>
> Modified:
> lld/trunk/ELF/Relocations.cpp
> lld/trunk/ELF/Relocations.h
>
> Modified: lld/trunk/ELF/Relocations.cpp
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.cpp?rev=288314&r1=288313&r2=288314&view=diff
> ==============================================================================
> --- lld/trunk/ELF/Relocations.cpp (original)
> +++ lld/trunk/ELF/Relocations.cpp Wed Nov 30 23:43:48 2016
> @@ -62,12 +62,10 @@ namespace lld {
> namespace elf {
>
> static bool refersToGotEntry(RelExpr Expr) {
> - return Expr == R_GOT || Expr == R_GOT_OFF || Expr == R_MIPS_GOT_LOCAL_PAGE ||
> - Expr == R_MIPS_GOT_OFF || Expr == R_MIPS_GOT_OFF32 ||
> - Expr == R_MIPS_TLSGD || Expr == R_MIPS_TLSLD ||
> - Expr == R_GOT_PAGE_PC || Expr == R_GOT_PC || Expr == R_GOT_FROM_END ||
> - Expr == R_TLSGD || Expr == R_TLSGD_PC || Expr == R_TLSDESC ||
> - Expr == R_TLSDESC_PAGE;
> + return isRelExprOneOf<R_GOT, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE, R_MIPS_GOT_OFF,
> + R_MIPS_GOT_OFF32, R_MIPS_TLSGD, R_MIPS_TLSLD,
> + R_GOT_PAGE_PC, R_GOT_PC, R_GOT_FROM_END, R_TLSGD,
> + R_TLSGD_PC, R_TLSDESC, R_TLSDESC_PAGE>(Expr);
> }
>
> static bool isPreemptible(const SymbolBody &Body, uint32_t Type) {
> @@ -302,16 +300,16 @@ template <class ELFT> static bool isAbso
> }
>
> static bool needsPlt(RelExpr Expr) {
> - return Expr == R_PLT_PC || Expr == R_PPC_PLT_OPD || Expr == R_PLT ||
> - Expr == R_PLT_PAGE_PC || Expr == R_THUNK_PLT_PC;
> + return isRelExprOneOf<R_PLT_PC, R_PPC_PLT_OPD, R_PLT, R_PLT_PAGE_PC,
> + R_THUNK_PLT_PC>(Expr);
> }
>
> // True if this expression is of the form Sym - X, where X is a position in the
> // file (PC, or GOT for example).
> static bool isRelExpr(RelExpr Expr) {
> - return Expr == R_PC || Expr == R_GOTREL || Expr == R_GOTREL_FROM_END ||
> - Expr == R_MIPS_GOTREL || Expr == R_PAGE_PC || Expr == R_RELAX_GOT_PC ||
> - Expr == R_THUNK_PC || Expr == R_THUNK_PLT_PC;
> + return isRelExprOneOf<R_PC, R_GOTREL, R_GOTREL_FROM_END, R_MIPS_GOTREL,
> + R_PAGE_PC, R_RELAX_GOT_PC, R_THUNK_PC, R_THUNK_PLT_PC>(
> + Expr);
> }
>
> template <class ELFT>
> @@ -320,12 +318,11 @@ static bool isStaticLinkTimeConstant(Rel
> InputSectionBase<ELFT> &S,
> typename ELFT::uint RelOff) {
> // These expressions always compute a constant
> - if (E == R_SIZE || E == R_GOT_FROM_END || E == R_GOT_OFF ||
> - E == R_MIPS_GOT_LOCAL_PAGE || E == R_MIPS_GOT_OFF ||
> - E == R_MIPS_GOT_OFF32 || E == R_MIPS_TLSGD || E == R_GOT_PAGE_PC ||
> - E == R_GOT_PC || E == R_PLT_PC || E == R_TLSGD_PC || E == R_TLSGD ||
> - E == R_PPC_PLT_OPD || E == R_TLSDESC_CALL || E == R_TLSDESC_PAGE ||
> - E == R_HINT || E == R_THUNK_PC || E == R_THUNK_PLT_PC)
> + if (isRelExprOneOf<R_SIZE, R_GOT_FROM_END, R_GOT_OFF, R_MIPS_GOT_LOCAL_PAGE,
> + R_MIPS_GOT_OFF, R_MIPS_GOT_OFF32, R_MIPS_TLSGD,
> + R_GOT_PAGE_PC, R_GOT_PC, R_PLT_PC, R_TLSGD_PC, R_TLSGD,
> + R_PPC_PLT_OPD, R_TLSDESC_CALL, R_TLSDESC_PAGE, R_HINT,
> + R_THUNK_PC, R_THUNK_PLT_PC>(E))
> return true;
>
> // These never do, except if the entire file is position dependent or if
> @@ -659,12 +656,12 @@ static void scanRelocs(InputSectionBase<
>
> // Ignore "hint" and TLS Descriptor call relocation because they are
> // only markers for relaxation.
> - if (Expr == R_HINT || Expr == R_TLSDESC_CALL)
> + if (isRelExprOneOf<R_HINT, R_TLSDESC_CALL>(Expr))
> continue;
>
> - if (needsPlt(Expr) || Expr == R_THUNK_ABS || Expr == R_THUNK_PC ||
> - Expr == R_THUNK_PLT_PC || refersToGotEntry(Expr) ||
> - !isPreemptible(Body, Type)) {
> + if (needsPlt(Expr) ||
> + isRelExprOneOf<R_THUNK_ABS, R_THUNK_PC, R_THUNK_PLT_PC>(Expr) ||
> + refersToGotEntry(Expr) || !isPreemptible(Body, Type)) {
> // If the relocation points to something in the file, we can process it.
> bool Constant =
> isStaticLinkTimeConstant<ELFT>(Expr, Type, Body, C, RI.r_offset);
>
> Modified: lld/trunk/ELF/Relocations.h
> URL: http://llvm.org/viewvc/llvm-project/lld/trunk/ELF/Relocations.h?rev=288314&r1=288313&r2=288314&view=diff
> ==============================================================================
> --- lld/trunk/ELF/Relocations.h (original)
> +++ lld/trunk/ELF/Relocations.h Wed Nov 30 23:43:48 2016
> @@ -73,6 +73,35 @@ enum RelExpr {
> R_TLSLD_PC,
> };
>
> +// Build a bitmask with one bit set for each RelExpr.
> +//
> +// Constexpr function arguments can't be used in static asserts, so we
> +// use template arguments to build the mask.
> +// But function template partial specializations don't exist (needed
> +// for base case of the recursion), so we need a dummy struct.
> +template <RelExpr... Exprs> struct RelExprMaskBuilder {
> + static inline uint64_t build() { return 0; }
> +};
> +
> +// Specialization for recursive case.
> +template <RelExpr Head, RelExpr... Tail>
> +struct RelExprMaskBuilder<Head, Tail...> {
> + static inline uint64_t build() {
> + static_assert(0 <= Head && Head < 64,
> + "RelExpr is too large for 64-bit mask!");
> + return (uint64_t(1) << Head) | RelExprMaskBuilder<Tail...>::build();
> + }
> +};
> +
> +// Return true if `Expr` is one of `Exprs`.
> +// There are fewer than 64 RelExpr's, so we can represent any set of
> +// RelExpr's as a constant bit mask and test for membership with a
> +// couple cheap bitwise operations.
> +template <RelExpr... Exprs> bool isRelExprOneOf(RelExpr Expr) {
> + assert(0 <= Expr && Expr < 64 && "RelExpr is too large for 64-bit mask!");
> + return (uint64_t(1) << Expr) & RelExprMaskBuilder<Exprs...>::build();
> +}
> +
> // Architecture-neutral representation of relocation.
> struct Relocation {
> RelExpr Expr;
>
>
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