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Rewrite cast->cast elimination code completely based on the information we

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actually care about.  Someday when the cast instruction is gone, we can do
better here, but this will do for now.  This implements
instcombine/cast.ll:test17/18 as well.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15018 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2004-07-20 00:59:32 +00:00
parent 6c08892814
commit bee7e7644b
1 changed files with 52 additions and 42 deletions
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94
lib/Transforms/Scalar/InstructionCombining.cpp
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@ -1938,6 +1938,29 @@ Instruction *InstCombiner::visitShiftInst(ShiftInst &I) {
return 0;
}
enum CastType {
Noop = 0,
Truncate = 1,
Signext = 2,
Zeroext = 3
};
/// getCastType - In the future, we will split the cast instruction into these
/// various types. Until then, we have to do the analysis here.
static CastType getCastType(const Type *Src, const Type *Dest) {
assert(Src->isIntegral() && Dest->isIntegral() &&
"Only works on integral types!");
unsigned SrcSize = Src->getPrimitiveSize()*8;
if (Src == Type::BoolTy) SrcSize = 1;
unsigned DestSize = Dest->getPrimitiveSize()*8;
if (Dest == Type::BoolTy) DestSize = 1;
if (SrcSize == DestSize) return Noop;
if (SrcSize > DestSize) return Truncate;
if (Src->isSigned()) return Signext;
return Zeroext;
}
// isEliminableCastOfCast - Return true if it is valid to eliminate the CI
// instruction.
@ -1954,50 +1977,37 @@ static inline bool isEliminableCastOfCast(const Type *SrcTy, const Type *MidTy,
// Allow free casting and conversion of sizes as long as the sign doesn't
// change...
if (SrcTy->isIntegral() && MidTy->isIntegral() && DstTy->isIntegral()) {
unsigned SrcSize = SrcTy->getPrimitiveSize();
unsigned MidSize = MidTy->getPrimitiveSize();
unsigned DstSize = DstTy->getPrimitiveSize();
CastType FirstCast = getCastType(SrcTy, MidTy);
CastType SecondCast = getCastType(MidTy, DstTy);
// Cases where we are monotonically decreasing the size of the type are
// always ok, regardless of what sign changes are going on.
//
if (SrcSize >= MidSize && MidSize >= DstSize)
// Capture the effect of these two casts. If the result is a legal cast,
// the CastType is stored here, otherwise a special code is used.
static const unsigned CastResult[] = {
// First cast is noop
0, 1, 2, 3,
// First cast is a truncate
1, 1, 4, 4, // trunc->extend is not safe to eliminate
// First cast is a sign ext
2, 5, 2, 4, // signext->trunc always ok, signext->zeroext never ok
// First cast is a zero ext
3, 5, 3, 3, // zeroext->trunc always ok
};
unsigned Result = CastResult[FirstCast*4+SecondCast];
switch (Result) {
default: assert(0 && "Illegal table value!");
case 0:
case 1:
case 2:
case 3:
// FIXME: in the future, when LLVM has explicit sign/zeroextends and
// truncates, we could eliminate more casts.
return (unsigned)getCastType(SrcTy, DstTy) == Result;
case 4:
return false; // Not possible to eliminate this here.
case 5:
// Sign or zero extend followed by truncate is always ok
return true;
// Cases where the source and destination type are the same, but the middle
// type is bigger are noops.
//
if (SrcSize == DstSize && MidSize > SrcSize)
return true;
// If we are monotonically growing, things are more complex.
//
if (SrcSize <= MidSize && MidSize <= DstSize) {
// We have eight combinations of signedness to worry about. Here's the
// table:
static const int SignTable[8] = {
// CODE, SrcSigned, MidSigned, DstSigned, Comment
1, // U U U Always ok
1, // U U S Always ok
3, // U S U Ok iff SrcSize != MidSize
3, // U S S Ok iff SrcSize != MidSize
0, // S U U Never ok
2, // S U S Ok iff MidSize == DstSize
1, // S S U Always ok
1, // S S S Always ok
};
// Choose an action based on the current entry of the signtable that this
// cast of cast refers to...
unsigned Row = SrcTy->isSigned()*4+MidTy->isSigned()*2+DstTy->isSigned();
switch (SignTable[Row]) {
case 0: return false; // Never ok
case 1: return true; // Always ok
case 2: return MidSize == DstSize; // Ok iff MidSize == DstSize
case 3: // Ok iff SrcSize != MidSize
return SrcSize != MidSize || SrcTy == Type::BoolTy;
default: assert(0 && "Bad entry in sign table!");
}
}
}