emitRetUT: cope with arguments overlapping with results (#3546)

In decodeFloat_Int# we have the C-- code:

    mp_tmp1  = Sp - WDS(1);
    mp_tmp_w = Sp - WDS(2);
    
    /* arguments: F1 = Float# */
    arg = F1;
    
    /* Perform the operation */
    foreign "C" __decodeFloat_Int(mp_tmp1 "ptr", mp_tmp_w "ptr", arg) [];
    
    /* returns: (Int# (mantissa), Int# (exponent)) */
    RET_NN(W_[mp_tmp1], W_[mp_tmp_w]);

Which all looks quite reasonable.  The problem is that RET_NN() might
assign the results to the stack (with an unregisterised back end), and
in this case the arguments to RET_NN() refer to the same stack slots
that will be assigned to.

The code generator should do the right thing here, but it wasn't - it
was assuming that it could assign the results sequentially.  A 1-line
fix to use emitSimultaneously rather than emitStmts (plus comments).

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/cmm/CmmParse.y b/compiler/cmm/CmmParse.y
index 2c7ffe2845..3cd6be97a2 100644
--- a/compiler/cmm/CmmParse.y
+++ b/compiler/cmm/CmmParse.y
@@ -978,7 +978,9 @@ emitRetUT :: [(CgRep,CmmExpr)] -> Code
 emitRetUT args = do
   tickyUnboxedTupleReturn (length args)  -- TICK
   (sp, stmts) <- pushUnboxedTuple 0 args
-  emitStmts stmts
+  emitSimultaneously stmts -- NB. the args might overlap with the stack slots
+                           -- or regs that we assign to, so better use
+                           -- simultaneous assignments here (#3546)
   when (sp /= 0) $ stmtC (CmmAssign spReg (cmmRegOffW spReg (-sp)))
   stmtC (CmmJump (entryCode (CmmLoad (cmmRegOffW spReg sp) bWord)) [])
   -- TODO (when using CPS): emitStmt (CmmReturn (map snd args))