/// composeSubRegIndices - Return the subregister index you get from composing
/// two subregister indices.
///
+ /// The special null sub-register index composes as the identity.
+ ///
/// If R:a:b is the same register as R:c, then composeSubRegIndices(a, b)
/// returns c. Note that composeSubRegIndices does not tell you about illegal
/// compositions. If R does not have a subreg a, or R:a does not have a subreg
/// ssub_0:S0 - ssub_3:S3 subregs.
/// If you compose subreg indices dsub_1, ssub_0 you get ssub_2.
///
- virtual unsigned composeSubRegIndices(unsigned a, unsigned b) const {
- // This default implementation is correct for most targets.
- return b;
+ unsigned composeSubRegIndices(unsigned a, unsigned b) const {
+ if (!a) return b;
+ if (!b) return a;
+ return composeSubRegIndicesImpl(a, b);
}
+protected:
+ /// Overridden by TableGen in targets that have sub-registers.
+ virtual unsigned composeSubRegIndicesImpl(unsigned, unsigned) const {
+ llvm_unreachable("Target has no sub-registers");
+ }
+
+public:
/// getCommonSuperRegClass - Find a common super-register class if it exists.
///
/// Find a register class, SuperRC and two sub-register indices, PreA and