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readme updates

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@47051 91177308-0d34-0410-b5e6-96231b3b80d8
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Nate Begeman 2008-02-13 07:06:12 +00:00
parent 261c704da1
commit b5041b3085
2 changed files with 20 additions and 0 deletions
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9
lib/Target/PowerPC/README_ALTIVEC.txt
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@ -177,3 +177,12 @@ which prevents the vnot pattern from matching.
//===----------------------------------------------------------------------===//
An alternative to the store/store/load approach for illegal insert element
lowering would be:
1. store element to any ol' slot
2. lvx the slot
3. lvsl 0; splat index; vcmpeq to generate a select mask
4. lvsl slot + x; vperm to rotate result into correct slot
5. vsel result together.

11
lib/Target/X86/README-SSE.txt
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@ -781,3 +781,14 @@ any number of 0.0 simultaneously. Currently we only use it for simple
insertions.
See comments in LowerINSERT_VECTOR_ELT_SSE4.
//===---------------------------------------------------------------------===//
On a random note, SSE2 should declare insert/extract of 2 x f64 as legal, not
Custom. All combinations of insert/extract reg-reg, reg-mem, and mem-reg are
legal, it'll just take a few extra patterns written in the .td file.
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.