On Wed, 2005-12-07 at 21:55 -0700, Roger Sayle wrote: > Hi Dan, > > On Wed, 7 Dec 2005, Daniel Berlin wrote: > > This is *already* wrong, AFAICT, because reg:QI 58 is uninitialized, and > > we are trying to use it's value. Why do we do this? > > This may have been a rhetorical question, but the middle-end's RTL > expanders are generating uses of uninitialized registers when writing > to bitfields of a structure that has been allocated to a register. > > In your example, the middle-end has decided that the struct "flags" > should be placed in (reg:QI 58). Then when the first statement > decides to initialize the single bit corresponding to field c to zero, > it uses the idiom of (set (reg:QI 58) (and:QI (reg:QI 58) (const_int))) > > The possible fixes to this are for the middle-end to generate either > code to initialize bitfield-containing structures when they're stored > in registers, i.e. something like (set (reg:QI 58) (const_int 0)) at > the start of the function, or to emit some form of nop_set or clobber > to make it explicit that the contents of register QI:58 are initially > undefined but may be used before becoming defined. > > Both of these should allow the middle-end's optimizers to do a better > job, for example, the and's and or's to set QI:58 to a specific value > should ideally be reduced to a single (set (reg:QI 58) (const_int foo)).
Exactly. It's not just that nothing apparently stops combine from doing the and + or -> or transformation, it's that if we had initialized it to 0, all the and's and or's would have been constant propagated away. We are only hurting ourselves by not emitting an explicit set *in this case* so that it knows the initial value. > > The potential concern with "initialization" is that this may result > in additional overhead, if the RTL optimizers can't determine that > an initialization is dead when the result is unused/cobbered. The new DCE should be able to do this in *all* cases where you've transformed the structure into a register. If it has no uses, it has no uses, and will go away.
