llvm-6502/lib/Transforms
Sanjoy Das 4db527032a [IndVars] Try to use existing values in RewriteLoopExitValues.
Summary:
In RewriteLoopExitValues, before expanding out an SCEV expression using
SCEVExpander, try to see if an existing LLVM IR expression already
computes the value we're interested in.  If so use that existing
expression.

Apart from reducing IndVars' reliance on the rest of the compilation
pipeline, this also prevents IndVars from concluding some expressions as
"high cost" when they're not.  For instance,
`InductiveRangeCheckElimination` often emits code of the following form:

```
len = umin(len_A, len_B)

loop:
  ...
  if (i++ < len)
    goto loop

outside_loop:
    use(i)
```

`SCEVExpander` refuses to rewrite the use of `i` in `outside_loop`,
since it thinks the value of `i` on loop exit, `len`, is a high cost
expansion since it contains an `umax` in it.  With this change,
`IndVars` can see that it can re-use `len` instead of creating a new
expression to compute `umin(len_A, len_B)`.

I considered putting this cleverness in `SCEVExpander`, but I was
worried that it may then have a deterimental effect on other passes
that use it.  So I decided it was better to just do this in the one
place where it seems like an obviously good idea, with the intent of
generalizing later if needed.

Reviewers: atrick, reames

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D10782

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241838 91177308-0d34-0410-b5e6-96231b3b80d8
2015-07-09 18:46:12 +00:00
..
Hello Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC) 2015-06-23 09:49:53 +00:00
InstCombine [InstCombine] call SimplifyICmpInst with correct context 2015-06-25 20:14:47 +00:00
Instrumentation [SanitizerCoverage] Don't add instrumentation to unreachable blocks. 2015-06-30 23:11:45 +00:00
IPO [llvm-extract] Drop comdats from declarations 2015-07-06 18:48:02 +00:00
ObjCARC Revert r240137 (Fixed/added namespace ending comments using clang-tidy. NFC) 2015-06-23 09:49:53 +00:00
Scalar [IndVars] Try to use existing values in RewriteLoopExitValues. 2015-07-09 18:46:12 +00:00
Utils [LoopUnroll] Use undef for phis with no value live 2015-07-01 05:38:07 +00:00
Vectorize [SLPVectorizer] Try different vectorization factors for store chains 2015-07-08 23:40:55 +00:00
CMakeLists.txt
LLVMBuild.txt
Makefile