Branchless arithmetic operations

While working on #9281 (closed) I noticed sub-optimal code generation for common arithmetic operations on Int. Below are some examples of code generation, where the branchless versions may be desirable on modern CPUs:

abs

-- base version
absI# :: Int# -> Int#
absI# i# = i'#
  where
    !(I# i'#) = abs (I# i#)

-- optimized version
optAbsI# :: Int# -> Int#
optAbsI# i# = (i# `xorI#` nsign) -# nsign
  where
    -- nsign = i# <# 0#
    nsign = uncheckedIShiftRA# i# (WORD_SIZE_IN_BITS# -# 1#)

results in

absI#_info:
_c1To:
	testq %r14,%r14
	jge _c1Tx
_c1Ty:
	movq %r14,%rbx
	negq %rbx
	jmp *(%rbp)
_c1Tx:
	movq %r14,%rbx
	jmp *(%rbp)


optAbsI#_info:
_c1SX:
	movq %r14,%rax
	sarq $63,%rax
	movq %r14,%rbx
	xorq %rax,%rbx
	subq %rax,%rbx
	jmp *(%rbp)

signum

sgnI# :: Int# -> Int#
sgnI# i# = i'#
  where
    !(I# i'#) = signum (I# i#)

optSgnI# :: Int# -> Int#
optSgnI# x# = (x# ># 0#) -# (x# <# 0#)

sgnI#_info:
_c27W:
        testq %r14,%r14
        jl _c283
_c284:
        testq %r14,%r14
        jne _c28a
_c28b:
        xorl %ebx,%ebx
        jmp *(%rbp)
_c283:
        movq $-1,%rbx
        jmp *(%rbp)
_c28a:
        movl $1,%ebx
        jmp *(%rbp)


optSgnI#_info:
_c26P:
        testq %r14,%r14
        setl %al
        movzbl %al,%eax
        testq %r14,%r14
        setg %bl
        movzbl %bl,%ebx
        subq %rax,%rbx
        jmp *(%rbp)

compare

cmpI# :: Int# -> Int# -> Int#
cmpI# x# y# = dataToTag# (compare (I# x#) (I# y#))

-- returns 0, 1 or 2, can be fed to tagToEnum# :: Ordering
optCmpI# :: Int# -> Int# -> Int#
optCmpI# x# y# = 1# +# (x# ># y#) -# (x# <# y#)

cmpI#_info:
_c25s:
	cmpq %rsi,%r14
	jl _c25z
_c25A:
	cmpq %rsi,%r14
	je _c25M
_c25N:
	movl $2,%ebx
	jmp *(%rbp)
_c25z:
	xorl %ebx,%ebx
	jmp *(%rbp)
_c25M:
	movl $1,%ebx
	jmp *(%rbp)


optCmpI#_info:
_c24N:
	cmpq %rsi,%r14
	setl %al
	movzbl %al,%eax
	movl $1,%ebx
	subq %rax,%rbx
	cmpq %rsi,%r14
	setg %al
	movq %rbx,%rcx
	movzbl %al,%ebx
	addq %rcx,%rbx
	jmp *(%rbp)

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (CodeGen)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	simonmar
Operating system
Architecture