Add docstring and tests

base/compiler/ssair/passes.jl

@@ -1963,6 +1963,18 @@ function perform_symbolic_evaluation(stmt::PhiNode, ssa_to_ssa, blockidx)
     return svec(key...)
 end
 
+"""
+    gnv!(ir::IRCode) -> newir::IRCode
+
+Global Value Numbering (GVN) pass.
+
+This is based on the NewGVN pass in LLVM.
+
+GVN partitions all the statements in the IR into congruence classes. All elements in a congruence class are guaranteed to be the same.
+
+It implements the RPO value numbering algorithm based on the paper "SCC based value numbering" by L. Taylor Simpson.
+The elimination step is based on the implemenation in LLVM's NewGVN pass.
+"""
 function gvn!(ir::IRCode)
     changed = true
     ssa_to_ssa = fill(0, length(ir.stmts.stmt)) # Map from ssa to ssa of equivalent value
@@ -1973,7 +1985,7 @@ function gvn!(ir::IRCode)
     while changed
         changed = false
 
-        # RPO Traversal
+        # Reverse Post Order traversal of dominator tree
         for (blockidx, block) in enumerate(ir.cfg.blocks), i in block.stmts
             if !(ir.stmts.stmt[i] isa Expr) & !(ir.stmts.stmt[i] isa PhiNode) 
                 ssa_to_ssa[i] = i

test/compiler/EscapeAnalysis/EAUtils.jl

@@ -63,8 +63,8 @@ import Core:
     CodeInstance, MethodInstance, CodeInfo
 import .CC:
     InferenceResult, OptimizationState, IRCode, copy as cccopy,
-    @timeit, convert_to_ircode, slot2reg, compact!, ssa_inlining_pass!, sroa_pass!,
-    adce_pass!, JLOptions, verify_ir, verify_linetable
+    @timeit, convert_to_ircode, slot2reg, compact!, ssa_inlining_pass!, gvn!, 
+    sroa_pass!, adce_pass!, JLOptions, verify_ir, verify_linetable
 import .EA: analyze_escapes, ArgEscapeCache, EscapeInfo, EscapeState, is_ipo_profitable
 
 # when working outside of Core.Compiler,
@@ -222,6 +222,7 @@ function run_passes_with_ea(interp::EscapeAnalyzer, ci::CodeInfo, sv::Optimizati
         interp.state = state
         interp.linfo = sv.linfo
     end
+    @timeit "GVN"       ir = gvn!(ir)
     @timeit "SROA"      ir = sroa_pass!(ir)
     @timeit "ADCE"      ir = adce_pass!(ir)
     @timeit "compact 3" ir = compact!(ir)

test/compiler/irpasses.jl

@@ -1452,3 +1452,25 @@ end
 fully_eliminated(; retval=Core.Argument(2)) do x::Float64
     return ifelse(isa(x, Float64), x, exp(x))
 end
+
+# Test that gvn! works on example from the paper "SCC based value numbering" by L. Taylor Simpson.
+let src = @eval Module() begin
+        function g()
+            x = 1
+            y = 1
+            while true
+                x += 1
+                y += 1
+            end
+        end
+        code_typed(g, Tuple{})[1][1]
+    end
+    nphi = 0
+    for stmt in src.code
+        if stmt isa Core.PhiNode
+            nphi += 1
+        end
+    end
+    @test nphi == 1
+    @test count(iscall((src, Base.add_int)), src.code) == 1
+end