Restructure dead argument elimination, try #3 :-)

Rewrite the DeadArgumentElimination pass, to use a more explicit tracking of
dependencies between return values and/or arguments. Also make the handling of
arguments and return values the same.

The pass now looks properly inside returned structs, but only at the first
level (ie, not inside nested structs).

This version fixed a few more bugs and was cleaned up a bit. It now passes all
of LLVM's testing, and should still pass SPEC2006. There is still a minor bug
with regard to returning nested structs. Since there is currently nothing that
emits such IR, I will fix that in a seperate commit (partly because it requires
a non-trivial fix).

llvm-svn: 53400

test/Transforms/DeadArgElim/multdeadretval.ll

@@ -3,7 +3,6 @@
 ; run instcombine to fold insert/extractvalue chains and we run dce to clean up
 ; any remaining dead stuff.
 ; RUN: llvm-as < %s | opt -deadargelim -instcombine -dce | llvm-dis | not grep i16
-; XFAIL: *
 
 define internal {i16, i32} @test(i16 %DEADARG) {
         %A = insertvalue {i16,i32} undef, i16 1, 0
@@ -18,6 +17,8 @@ define internal {i32, i16} @test2() {
         ret {i32,i16} %B
 }
 
+; Dead argument, used to check if the second result of test2 is dead even when
+; it's used as a dead argument
 define internal i32 @test3(i16 %A) {
         %ret = call {i16, i32} @test( i16 %A )                ; <i32> [#uses=0]
         %DEAD = extractvalue {i16, i32} %ret, 0
@@ -29,6 +30,14 @@ define internal i16 @test4() {
         ret i16 0
 }
 
+; Multiple return values, multiple live return values
+define internal {i32, i32, i16} @test5() {
+        %A = insertvalue {i32,i32,i16} undef, i32 1, 0
+        %B = insertvalue {i32,i32,i16} %A, i32 2, 1
+        %C = insertvalue {i32,i32,i16} %B, i16 3, 2
+        ret {i32, i32, i16} %C
+}
+
 define i32 @main() {
         %ret = call {i32, i16} @test2()                ; <i32> [#uses=1]
         %LIVE = extractvalue {i32, i16} %ret, 0
@@ -36,5 +45,11 @@ define i32 @main() {
         %Y = add i32 %LIVE, -123           ; <i32> [#uses=1]
         %LIVE2 = call i32 @test3(i16 %DEAD)                ; <i32> [#uses=1]
         %Z = add i32 %LIVE2, %Y           ; <i32> [#uses=1]
-        ret i32 %Z
+        %ret1 = call { i32, i32, i16 } @test5 ()
+        %LIVE3 = extractvalue { i32, i32, i16} %ret1, 0
+        %LIVE4 = extractvalue { i32, i32, i16} %ret1, 1
+        %DEAD2 = extractvalue { i32, i32, i16} %ret1, 2
+        %V = add i32 %LIVE3, %LIVE4
+        %W = add i32 %Z, %V
+        ret i32 %W
 }