[RFC] nativeGen: Add support for MO_SS_Conv_W32_W64 on i386

This is required by D4288. However, this only handles i386; we will
likely also need to do the same for PPC and SPARC, lest they break when
D4288 is re-merged.

Test Plan: Validate

Reviewers: simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4362

compiler/nativeGen/X86/CodeGen.hs

@@ -468,6 +468,20 @@ iselExpr64 (CmmMachOp (MO_UU_Conv _ W64) [expr]) = do
                           r_dst_lo
             )
 
+iselExpr64 (CmmMachOp (MO_SS_Conv W32 W64) [expr]) = do
+     fn <- getAnyReg expr
+     r_dst_lo <-  getNewRegNat II32
+     let r_dst_hi = getHiVRegFromLo r_dst_lo
+         code = fn r_dst_lo
+     return (
+             ChildCode64 (code `snocOL`
+                          MOV II32 (OpReg r_dst_lo) (OpReg eax) `snocOL`
+                          CLTD II32 `snocOL`
+                          MOV II32 (OpReg eax) (OpReg r_dst_lo) `snocOL`
+                          MOV II32 (OpReg edx) (OpReg r_dst_hi))
+                          r_dst_lo
+            )
+
 iselExpr64 expr
    = pprPanic "iselExpr64(i386)" (ppr expr)