[GC] Remove redundant entiries in stackmap section (and test it this time)

This is a reimplementation of the optimization removed in D75964. The actual spill/fill optimization is handled by D76013, this one just worries about reducing the stackmap section size itself by eliminating redundant entries. As noted in the comments, we could go a lot further here, but avoiding the degenerate invoke case as we did before is probably "enough" in practice.

Differential Revision: https://reviews.llvm.org/D76021

Author: preames

llvm/lib/CodeGen/SelectionDAG/StatepointLowering.cpp

@@ -603,7 +603,7 @@ SDValue SelectionDAGBuilder::LowerAsSTATEPOINT(
   // Clear state
   StatepointLowering.startNewStatepoint(*this);
   assert(SI.Bases.size() == SI.Ptrs.size() &&
-         SI.Ptrs.size() == SI.GCRelocates.size());
+         SI.Ptrs.size() <= SI.GCRelocates.size());
 
 #ifndef NDEBUG
   for (auto *Reloc : SI.GCRelocates)
@@ -823,10 +823,29 @@ SelectionDAGBuilder::LowerStatepoint(ImmutableStatepoint ISP,
                            ISP.getNumCallArgs(), ActualCallee,
                            ISP.getActualReturnType(), false /* IsPatchPoint */);
 
+  // There may be duplication in the gc.relocate list; such as two copies of
+  // each relocation on normal and exceptional path for an invoke.  We only
+  // need to spill once and record one copy in the stackmap, but we need to
+  // reload once per gc.relocate.  (Dedupping gc.relocates is trickier and best
+  // handled as a CSE problem elsewhere.)
+  // TODO: There a couple of major stackmap size optimizations we could do
+  // here if we wished.
+  // 1) If we've encountered a derived pair {B, D}, we don't need to actually
+  // record {B,B} if it's seen later.
+  // 2) Due to rematerialization, actual derived pointers are somewhat rare;
+  // given that, we could change the format to record base pointer relocations
+  // separately with half the space. This would require a format rev and a
+  // fairly major rework of the STATEPOINT node though.
+  SmallSet<SDValue, 8> Seen;
   for (const GCRelocateInst *Relocate : ISP.getRelocates()) {
     SI.GCRelocates.push_back(Relocate);
-    SI.Bases.push_back(Relocate->getBasePtr());
-    SI.Ptrs.push_back(Relocate->getDerivedPtr());
+
+    SDValue BaseSD = getValue(Relocate->getBasePtr());
+    SDValue DerivedSD = getValue(Relocate->getDerivedPtr());
+    if (Seen.insert(DerivedSD).second) {
+      SI.Bases.push_back(Relocate->getBasePtr());
+      SI.Ptrs.push_back(Relocate->getDerivedPtr());
+    }
   }
 
   SI.GCArgs = ArrayRef<const Use>(ISP.gc_args_begin(), ISP.gc_args_end());

llvm/test/CodeGen/X86/statepoint-stackmap-size.ll

@@ -0,0 +1,22 @@
+; RUN: llc  -verify-machineinstrs < %s | fgrep -A 10000 .llvm_stackmaps | wc -l | FileCheck %s
+
+; Without removal of duplicate entries, the size is 62 lines
+; CHECK: 50
+
+target triple = "x86_64-pc-linux-gnu"
+
+declare void @func()
+
+define i1 @test1(i32 addrspace(1)* %arg) gc "statepoint-example" {
+entry:
+  %safepoint_token = call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 0, i32 0, void ()* @func, i32 0, i32 0, i32 0, i32 0, i32 addrspace(1)* %arg)
+  %reloc1 = call i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(token %safepoint_token,  i32 7, i32 7)
+  %reloc2 = call i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(token %safepoint_token,  i32 7, i32 7)
+  %cmp1 = icmp eq i32 addrspace(1)* %reloc1, null
+  %cmp2 = icmp eq i32 addrspace(1)* %reloc2, null
+  %cmp = and i1 %cmp1, %cmp2
+  ret i1 %cmp
+}
+
+declare token @llvm.experimental.gc.statepoint.p0f_isVoidf(i64, i32, void ()*, i32, i32, ...)
+declare i32 addrspace(1)* @llvm.experimental.gc.relocate.p1i32(token, i32, i32)