Improve cmm/assembly for pattern matches with two constants.

For a pattern like:

f_test :: Int#->Int#
f_test a = case a of
    1# -> 33#
    7# -> 34#
     _ -> -1#

GHC currently generates code that works best if the default branch is taken most often.

Pseudo-Code

if (a >= 8)
    return -1;
else {
    if (a < 7) {
        if(a != 1)
            return -1;
        else
            return 33;
    } 
    else
        return 34;
}

CMM:

       c1cr: // global
           if (%MO_S_Ge_W64(R2, 8)) goto c1co; else goto u1cu;
       u1cu: // global
           if (%MO_S_Lt_W64(R2, 7)) goto u1cv; else goto c1cq;
       u1cv: // global
           if (R2 != 1) goto c1co; else goto c1cp;
       c1co: // global
           R1 = (-1);
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
       c1cp: // global
           R1 = 33;
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
       c1cq: // global
           R1 = 34;
           call (P64[Sp])(R1) args: 8, res: 0, upd: 8;
     }

Wouldn't the following be better?

if(a == 1)
    return 33
else if(a == 7)
    return 34
else
    return -1

I would expect that to

• Improve the cases:

  * a = 1

  * a < 7

• Be the same for:

  * a = 7

• Be worse for

  * a > 8

This would be especially nice for the cases where the default branch is raising a pattern match exception. Which is a code path I wouldn't expect to be taken often nor be very performance sensitive.

Even if we keep the current logic there is room for improvement. GHC currently creates the following assembly:

_c1cr:
	cmpq $8,%r14
	jge _c1co
_u1cu:
	cmpq $7,%r14
	jl _u1cv
_c1cq:
	movl $34,%ebx
	jmp *(%rbp)
_u1cv:
	cmpq $1,%r14
	jne _c1co
_c1cp:
	movl $33,%ebx
	jmp *(%rbp)
_c1co:
	movq $-1,%rbx
	jmp *(%rbp)

It would be nice if we could remove one comparison at least.

_c1cr:
	cmpq $7,%r14
	jg _c1co
_u1cu:
	;No longer neccesary cmpq $7,%r14
	jl _u1cv
_c1cq:
	movl $34,%ebx
	jmp *(%rbp)
_u1cv:
	cmpq $1,%r14
	jne _c1co
_c1cp:
	movl $33,%ebx
	jmp *(%rbp)
_c1co:
	movq $-1,%rbx
	jmp *(%rbp)

Trac metadata

Trac field	Value
Version	8.2.2
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
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