Eliminate stack adjustment for float-returning functions

src/IceTargetLoweringX86BaseImpl.h

Index: src/IceTargetLoweringX86BaseImpl.h
diff --git a/src/IceTargetLoweringX86BaseImpl.h b/src/IceTargetLoweringX86BaseImpl.h
index 8f372196779cc6b3c068655afb1ccd4ff3948165..066bffc08a9da9626d6753302d5783c834cc51aa 100644
--- a/src/IceTargetLoweringX86BaseImpl.h
+++ b/src/IceTargetLoweringX86BaseImpl.h
@@ -5198,6 +5198,13 @@ template <class Machine>
 void TargetX86Base<Machine>::lowerUnreachable(
     const InstUnreachable * /*Inst*/) {
   _ud2();
+  // Add a fake use of esp to make sure esp stays alive for the entire

[Jim Stichnoth, 2015/11/13 23:30:33]

Could you factor this code and its lowerRet clone into a private helper
function?  Call it something like keepEspLiveAtExit().

[sehr, 2015/11/14 00:30:56]

Done.

+  // function. Otherwise post-call esp adjustments get dead-code eliminated.
+  // TODO: Are there more places where the fake use should be inserted? E.g.

[Jim Stichnoth, 2015/11/13 23:30:33]

Remove this TODO because I'm pretty sure this covers everything by now.  The
non-returning test case seems to be fine.

[sehr, 2015/11/14 00:30:56]

Done.

+  // "void f(int n){while(1) g(n);}" may not have an unreachable instruction.
+  Variable *esp =
+      Func->getTarget()->getPhysicalRegister(Traits::RegisterSet::Reg_esp);

[Jim Stichnoth, 2015/11/13 23:30:33]

Use getStackReg() instead of Reg_esp.

BTW, getStackReg() will probably need to be put into Traits and redirected to
Reg_rsp under x86-64, but at least that's one fewer place to track down.

[sehr, 2015/11/14 00:30:56]

Done.

+  Context.insert(InstFakeUse::create(Func, esp));
 }
 
 template <class Machine>