[Subzero][MIPS32] Implements 64-bit shl, lshr, ashr for MIPS

src/IceInstMIPS32.h

Index: src/IceInstMIPS32.h
diff --git a/src/IceInstMIPS32.h b/src/IceInstMIPS32.h
index a4660592747bed11cde48086a3f3338e1c19dbcf..237da019b837b5ab45939cee3baee94ada80b098 100644
--- a/src/IceInstMIPS32.h
+++ b/src/IceInstMIPS32.h
@@ -1035,6 +1035,7 @@ private:
       : InstMIPS32(Func, K, 2, Dest) {
     addSource(Src);
     addSource(FCC);
+    setDestRedefined();

[Jim Stichnoth, 2016/09/21 17:08:20]

See my comment in
https://codereview.chromium.org/2357143002/diff/1/src/IceTargetLoweringMIPS32.h

[sagar.thakur, 2016/09/22 14:43:50]

Done.

   }
 
   static const char *Opcode;
@@ -1084,7 +1085,7 @@ using InstMIPS32Mflo = InstMIPS32UnaryopGPR<InstMIPS32::Mflo>;
 using InstMIPS32Mov_d = InstMIPS32TwoAddrFPR<InstMIPS32::Mov_d>;
 using InstMIPS32Mov_s = InstMIPS32TwoAddrFPR<InstMIPS32::Mov_s>;
 using InstMIPS32Movf = InstMIPS32MovConditional<InstMIPS32::Movf>;
-using InstMIPS32Movn = InstMIPS32ThreeAddrGPR<InstMIPS32::Movn>;
+using InstMIPS32Movn = InstMIPS32MovConditional<InstMIPS32::Movn>;

[obucinac, 2016/09/21 12:47:49]

Why is this changed? MovConditional is used by FP movs which are using Floating
Point Condtion Codes for decision. MOVN and MOVZ are using value from GPR for
decision.

[Jim Stichnoth, 2016/09/23 14:10:37]

Is there an answer for this question?  (I'm not familiar enough to weight in.)

[obucinac, 2016/09/23 15:59:45]

The main difference  is that MovConditional has no Dest, and ThreeAddrGPR has. I
am concerned that this change may affect propper liveness analisys.

[Jim Stichnoth, 2016/09/23 18:49:11]

I've looked through this more carefully.  I think the original code is wrong and
the proposed change is correct.

If I understand correctly:
  movn A, B, C
sets A to either A or B, based on the value of C.  In this case, it is correct
to have A as both the dest and one of the src operands, and this is what
InstMIPS32MovConditional does.

On the other hand, if the instruction were to set A based on just B and C, and
completely ignored the old value in A, then ThreeAddrGPR would be more
appropriate.

 using InstMIPS32Movn_d = InstMIPS32ThreeAddrGPR<InstMIPS32::Movn_d>;
 using InstMIPS32Movn_s = InstMIPS32ThreeAddrGPR<InstMIPS32::Movn_s>;
 using InstMIPS32Movt = InstMIPS32MovConditional<InstMIPS32::Movt>;