Use appropriatly sized comparison instruction for small values.

GHC currently defaults all comparisons originating from Cmm switch statements to 64bit on x64.

This incurs a small overhead in instruction size. Fixing this manually gave a speedup of ~1,5% in microbenchmarks.

In detail we generate Cmm of the form:

      _s8Dg::P64 = R1;
      _c8EF::P64 = _s8Dg::P64 & 7;
      switch [1 .. 2] _c8EF::P64 {
          case 1 : goto c8Ey;
          case 2 : goto c8EC;
      }

Which results in assembly like:

	andl $7,%ebx
	cmpq $1,%rbx

It's obvious that c8EF fits into a byte, but is sized up to 64 bits. Changing this would enable us to use cmpl instead of cmpq and save us a byte on each comparison.

While this isn't major in my microbenchmarks it resultet in a speedup of ~1,5% for such constructs in inner loops.

Trac metadata

Trac field	Value
Version
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture