Commit 31c260f3 authored by Herbert Valerio Riedel's avatar Herbert Valerio Riedel 🕺 Committed by Ben Gamari

Add ptr-eq short-cut to `compareByteArrays#` primitive

This is an obvious optimisation whose overhead is neglectable but
which significantly simplifies the common uses of `compareByteArrays#`
which would otherwise require to make *careful* use of
`reallyUnsafePtrEquality#` or (equally fragile) `byteArrayContents#`
which can result in less optimal assembler code being generated.

Test Plan: carefully examined generated cmm/asm code; validate via phab

Reviewers: alexbiehl, bgamari, simonmar

Reviewed By: bgamari, simonmar

Subscribers: rwbarton, thomie, carter

Differential Revision:
parent e7dcc708
......@@ -1746,8 +1746,51 @@ doCompareByteArraysOp res ba1 ba1_off ba2 ba2_off n = do
dflags <- getDynFlags
ba1_p <- assignTempE $ cmmOffsetExpr dflags (cmmOffsetB dflags ba1 (arrWordsHdrSize dflags)) ba1_off
ba2_p <- assignTempE $ cmmOffsetExpr dflags (cmmOffsetB dflags ba2 (arrWordsHdrSize dflags)) ba2_off
-- short-cut in case of equal pointers avoiding a costly
-- subroutine call to the memcmp(3) routine; the Cmm logic below
-- results in assembly code being generated for
-- cmpPrefix10 :: ByteArray# -> ByteArray# -> Int#
-- cmpPrefix10 ba1 ba2 = compareByteArrays# ba1 0# ba2 0# 10#
-- that looks like
-- leaq 16(%r14),%rax
-- leaq 16(%rsi),%rbx
-- xorl %ecx,%ecx
-- cmpq %rbx,%rax
-- je l_ptr_eq
-- ; NB: the common case (unequal pointers) falls-through
-- ; the conditional jump, and therefore matches the
-- ; usual static branch prediction convention of modern cpus
-- subq $8,%rsp
-- movq %rbx,%rsi
-- movq %rax,%rdi
-- movl $10,%edx
-- xorl %eax,%eax
-- call memcmp
-- addq $8,%rsp
-- movslq %eax,%rax
-- movq %rax,%rcx
-- l_ptr_eq:
-- movq %rcx,%rbx
-- jmp *(%rbp)
l_ptr_eq <- newBlockId
l_ptr_ne <- newBlockId
emit (mkAssign (CmmLocal res) (zeroExpr dflags))
emit (mkCbranch (cmmEqWord dflags ba1_p ba2_p)
l_ptr_eq l_ptr_ne (Just False))
emitLabel l_ptr_ne
emitMemcmpCall res ba1_p ba2_p n 1
emitLabel l_ptr_eq
-- ----------------------------------------------------------------------------
-- Copying byte arrays
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