[llgo] Optimize comparisons to empty string.

[pcc]

Summary

Geo-mean performance improvement of 0.2% (-0.3% - 0.9% @ 95% CI).

[axw]

LGTM.

irgen/strings.go

29–31

result := fr.builder.CreateIsNull(len, "")
?

[pcc]

Closed by commit rL223728 (authored by @pcc).

irgen/strings.go

29–31

Done.

irgen/strings.go

import (
   "llvm.org/llvm/bindings/go/llvm"
 )
 
 func (fr *frame) concatenateStrings(lhs, rhs *govalue) *govalue {
   result := fr.runtime.stringPlus.call(fr, lhs.value, rhs.value)
   return newValue(result[0], types.Typ[types.String])
 }
 
+func (fr *frame) compareStringEmpty(v llvm.Value) *govalue {
+  len := fr.builder.CreateExtractValue(v, 1, "")
+  zero := llvm.ConstNull(len.Type())
+  result := fr.builder.CreateICmp(llvm.IntEQ, len, zero, "")
+  result = fr.builder.CreateZExt(result, llvm.Int8Type(), "")
+  return newValue(result, types.Typ[types.Bool])
+}
+
 func (fr *frame) compareStrings(lhs, rhs *govalue, op token.Token) *govalue {
+  if op == token.EQL {
+    if lhs.value.IsNull() {
+      return fr.compareStringEmpty(rhs.value)
+    }
+    if rhs.value.IsNull() {
+      return fr.compareStringEmpty(lhs.value)
+    }
+  }
+
   result := fr.runtime.strcmp.call(fr, lhs.value, rhs.value)[0]
   zero := llvm.ConstNull(fr.types.inttype)
   var pred llvm.IntPredicate
   switch op {
   case token.EQL:
     pred = llvm.IntEQ
   case token.LSS:
     pred = llvm.IntSLT