89f7805c2e
Preliminary compiler support for merging/overlapping stack slots of local variables whose access patterns are disjoint. This patch includes changes in AllocFrame to do the actual merging/overlapping based on information returned from a new liveness.MergeLocals helper. The MergeLocals helper identifies candidates by looking for sets of AUTO variables that either A) have the same size and GC shape (if types contain pointers), or B) have the same size (but potentially different types as long as those types have no pointers). Variables must be greater than (3*types.PtrSize) in size to be considered for merging. After forming candidates, MergeLocals collects variables into "can be overlapped" equivalence classes or partitions; this process is driven by an additional liveness analysis pass. Ideally it would be nice to move the existing stackmap liveness pass up before AllocFrame and "widen" it to include merge candidates so that we can do just a single liveness as opposed to two passes, however this may be difficult given that the merge-locals liveness has to take into account writes corresponding to dead stores. This patch also required a change to the way ssa.OpVarDef pseudo-ops are generated; prior to this point they would only be created for variables whose type included pointers; if stack slot merging is enabled then the ssagen code creates OpVarDef ops for all auto vars that are merge candidates. Note that some temporaries created late in the compilation process (e.g. during ssa backend) are difficult to reason about, especially in cases where we take the address of a temp and pass it to the runtime. For the time being we mark most of the vars created post-ssagen as "not a merge candidate". Stack slot merging for locals/autos is enabled by default if "-N" is not in effect, and can be disabled via "-gcflags=-d=mergelocals=0". Fixmes/todos/restrictions: - try lowering size restrictions - re-evaluate the various skips that happen in SSA-created autotmps Fixes #62737. Updates #65532. Updates #65495. Cq-Include-Trybots: luci.golang.try:gotip-linux-amd64-longtest Change-Id: Ibc22e8a76c87e47bc9fafe4959804d9ea923623d Reviewed-on: https://go-review.googlesource.com/c/go/+/553055 Reviewed-by: Cherry Mui <cherryyz@google.com> LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> |
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