Subzero: Produce actually correct code in -asm-verbose mode.

src/IceInstX8632.cpp

Index: src/IceInstX8632.cpp
diff --git a/src/IceInstX8632.cpp b/src/IceInstX8632.cpp
index 15e024915f5a405f9bd90c39aa8a7a8f681763f7..7070e27cfbda567ca3fbc138e3988edc0ca2a1fb 100644
--- a/src/IceInstX8632.cpp
+++ b/src/IceInstX8632.cpp
@@ -318,7 +318,14 @@ InstX8632Fstp::InstX8632Fstp(Cfg *Func, Variable *Dest)
     : InstX8632(Func, InstX8632::Fstp, 0, Dest) {}
 
 InstX8632Pop::InstX8632Pop(Cfg *Func, Variable *Dest)
-    : InstX8632(Func, InstX8632::Pop, 0, Dest) {}
+    : InstX8632(Func, InstX8632::Pop, 0, Dest) {
+  // A pop instruction affects the stack pointer and so it should not
+  // be allowed to be automatically dead-code eliminated.  (The
+  // corresponding push instruction doesn't need this treatment
+  // because it has no dest variable and therefore won't be dead-code
+  // eliminated.)  This is needed for asm-verbose mode.

[jvoung (off chromium), 2015/04/29 16:41:35]

Could this be described more generically (vs "for asm-verbose")? Seems like it
is a general problem if liveness again after the instructions are added.

[Jim Stichnoth, 2015/04/29 17:15:27]

OK, I added "late-stage liveness analysis" to the asm-verbose comments.

For all of these additions, I wanted to leave a git grep'able comment because of
the weirdness of these situations, in case someone in the future needs to look
at them for inspiration.  Hence 'asm-verbose' in the comments.

+  HasSideEffects = true;
+}
 
 InstX8632Push::InstX8632Push(Cfg *Func, Variable *Source)
     : InstX8632(Func, InstX8632::Push, 1, nullptr) {