Hi,
I have some simple questions I hope somebody can answer.
Given
WN* op1;
WN* op2;
With op1 being
U8U8LDID
U4U8CVT
and op2 being
I4I4LDID
INEG
I wrote
op2 = WN_Type_Conversion(op2, MTYPE_U4);
op1 = WN_Add(MTYPE_U4, op2, op1);
I was surprised to see the WHIRL as
U8U8LDID
U4U8CVT
I4I4LDID
I4SUB
I really wanted an unsigned since I want to zero-extend that to an MTYPE_U8
with a U8U4CVT, but with the I4SUB, my code trips into generating an U8I4CVT
which sign-extends, not zero-extends.
I even tried a WN_Type_Conversion after the WN_Add but that didn't do much.
Tracking it down, I have two questions/concerns:
- WN_Int_Type_Conversion (called from WN_Type_Conversion) pretty much treats I4s
and U4s as identical. So when I called WN_Type_Conversion on 'op2', it just
returned the INEG node.
- WN_Add eventually calls simp_add_sub() inside wn_simp_code.h. It
wants to fold
the INEG away by turning the ADD into a SUB. No problem on the
surface, but I found
code near the top of simp_add_sub()
if (issub) {
subop = opc;
addop = OPC_FROM_OPR(OPR_ADD,ty);
} else {
addop = opc;
subop = OPC_FROM_OPR(OPR_SUB,ty);
#ifdef TARG_X8664 // bug 11400: if any operand is signed type, make SUB signed
if (MTYPE_signed(SIMPNODE_rtype(k0)) || MTYPE_signed(SIMPNODE_rtype(k1)))
subop = OPC_FROM_OPR(OPR_SUB, Mtype_TransferSign(MTYPE_I4, ty));
#endif
so later when the code simplifies the ADD/NEG into a SUB, you get a MTYPE_I4,
not the requested MTYPE_U4. (I'm on a X8664 target of course)
} else if (SIMPNODE_operator(k1)==OPR_NEG) {
SHOW_RULE(" x + (-y) ");
r = SIMPNODE_SimpCreateExp2(subop,k0,SIMPNODE_kid0(k1));
SIMP_DELETE(k1);
So finally to my questions:
Seems I cannot trust WN_Add to honor my MTYPE_U4 requests for X8664 targets?
Why just for X8664? If it is a matter of algebraic correctness, why not all
targets? If it is some assumption on the eventual generated code, seems risky.
WN_Int_Type_Conversion doesn't seem to help me out of this situation with its
I4/U4 equality.
I noticed what wgen avoids WN_Type_Conversion and calls WN_Cvt itself when
converts are needed. Is WN_Type_Conversion only for certain situations and
not for general use?
Thanks!
John
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