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Diffstat (limited to 'lib/Target/X86/README-SSE.txt')
| -rw-r--r-- | lib/Target/X86/README-SSE.txt | 30 |
1 files changed, 30 insertions, 0 deletions
diff --git a/lib/Target/X86/README-SSE.txt b/lib/Target/X86/README-SSE.txt index acfe411623..7248a57df3 100644 --- a/lib/Target/X86/README-SSE.txt +++ b/lib/Target/X86/README-SSE.txt @@ -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. |