From 634494366c515a89c4747d8a68a8da9218bb4969 Mon Sep 17 00:00:00 2001 From: Andrew Burgess Date: Thu, 29 Nov 2018 15:51:58 +0000 Subject: [PATCH] gdb/riscv: Create each unique target description only once GDB relies on the fact that if two target descriptions have the same contents, then they will be the same object instance (having the same address). One place where this is a requirement is in GDBARCH_LIST_LOOKUP_BY_INFO which is used to find previously created gdbarch objects. In GDBARCH_LIST_LOOKUP_BY_INFO a pointer comparison is made on the gdbarch's target description, if the pointers are different then it is assumed the gdbarches have different, non-compatible target descriptions. Previously we would create duplicate target descriptions in the belief that RISCV_GDBARCH_INIT would spot this duplication and discard the second instance. However, this was incorrect, and instead we ended up creating duplicate gdbarch objects. With this commit every unique feature set will create one and only one target description, the feature set and resulting target description is then cached so that the same target description object can be returned later. Many other target avoid this problem by creating a small number of named target descriptions, and returning one of these. However, we currently have 8 possible target descriptions (32 vs 64 bit for x-reg and f-reg, and h/w or s/w float abi) and creating each of these just to avoid a dynamic cache seems pointless. gdb/ChangeLog: * arch/riscv.h (riscv_gdbarch_features::hash): New method. * arch/riscv.c (struct riscv_gdbarch_features_hasher): New. (riscv_tdesc_cache): New global. (riscv_create_target_description): Look in the cache before creating a new target description. --- gdb/ChangeLog | 8 ++++++++ gdb/arch/riscv.c | 31 +++++++++++++++++++++++++++++++ gdb/arch/riscv.h | 9 +++++++++ 3 files changed, 48 insertions(+) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 306e1a1132c..666f0783e29 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,11 @@ +2018-11-30 Andrew Burgess + + * arch/riscv.h (riscv_gdbarch_features::hash): New method. + * arch/riscv.c (struct riscv_gdbarch_features_hasher): New. + (riscv_tdesc_cache): New global. + (riscv_create_target_description): Look in the cache before + creating a new target description. + 2018-11-30 Andrew Burgess * arch/riscv.h (riscv_gdb_features::operator==): New. diff --git a/gdb/arch/riscv.c b/gdb/arch/riscv.c index cb715fabb1f..81fd2f2f60f 100644 --- a/gdb/arch/riscv.c +++ b/gdb/arch/riscv.c @@ -18,17 +18,45 @@ #include "common-defs.h" #include "riscv.h" #include +#include #include "../features/riscv/32bit-cpu.c" #include "../features/riscv/64bit-cpu.c" #include "../features/riscv/32bit-fpu.c" #include "../features/riscv/64bit-fpu.c" +/* Wrapper used by std::unordered_map to generate hash for feature set. */ +struct riscv_gdbarch_features_hasher +{ + std::size_t + operator() (const riscv_gdbarch_features &features) const noexcept + { + return features.hash (); + } +}; + +/* Cache of previously seen target descriptions, indexed by the feature set + that created them. */ +static std::unordered_map riscv_tdesc_cache; + /* See arch/riscv.h. */ const target_desc * riscv_create_target_description (struct riscv_gdbarch_features features) { + /* Have we seen this feature set before? If we have return the same + target description. GDB expects that if two target descriptions are + the same (in content terms) then they will actually be the same + instance. This is important when trying to lookup gdbarch objects as + GDBARCH_LIST_LOOKUP_BY_INFO performs a pointer comparison on target + descriptions to find candidate gdbarch objects. */ + const auto it = riscv_tdesc_cache.find (features); + if (it != riscv_tdesc_cache.end ()) + return it->second; + + /* Now we should create a new target description. */ target_desc *tdesc = allocate_target_description (); #ifndef IN_PROCESS_AGENT @@ -65,5 +93,8 @@ riscv_create_target_description (struct riscv_gdbarch_features features) else if (features.flen == 8) regnum = create_feature_riscv_64bit_fpu (tdesc, regnum); + /* Add to the cache. */ + riscv_tdesc_cache.emplace (features, tdesc); + return tdesc; } diff --git a/gdb/arch/riscv.h b/gdb/arch/riscv.h index be41828eff6..79f5ec622d4 100644 --- a/gdb/arch/riscv.h +++ b/gdb/arch/riscv.h @@ -66,6 +66,15 @@ struct riscv_gdbarch_features { return !((*this) == rhs); } + + /* Used by std::unordered_map to hash feature sets. */ + std::size_t hash () const noexcept + { + std::size_t val = ((xlen & 0x1f) << 6 + | (flen & 0x1f) << 1 + | (hw_float_abi ? 1 : 0)); + return val; + } }; /* Create and return a target description that is compatible with -- 2.30.2