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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47939 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chris Lattner 2008-03-05 07:22:39 +00:00
parent ec4a5672f8
commit 063d825208
1 changed files with 30 additions and 0 deletions
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30
lib/Target/X86/README-SSE.txt
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@ -748,3 +748,33 @@ Note: this is not a code quality issue; the custom lowered code happens to be
right, but we shouldn't have to custom lower anything. This is probably related
to <2 x i64> ops being so bad.
//===---------------------------------------------------------------------===//
'select' on vectors and scalars could be a whole lot better. We currently
lower them to conditional branches. On x86-64 for example, we compile this:
double test(double a, double b, double c, double d) { return a<b ? c : d; }
to:
_test:
ucomisd %xmm0, %xmm1
ja LBB1_2 # entry
LBB1_1: # entry
movapd %xmm3, %xmm2
LBB1_2: # entry
movapd %xmm2, %xmm0
ret
instead of:
_test:
cmpltsd %xmm1, %xmm0
andpd %xmm0, %xmm2
andnpd %xmm3, %xmm0
orpd %xmm2, %xmm0
ret
For unpredictable branches, the later is much more efficient. This should
just be a matter of having scalar sse map to SELECT_CC and custom expanding
or iseling it.