Use a loop to simplify the runtime unrolling prologue.

Runtime unrolling will create a prologue to execute the extra
iterations which is can't divided by the unroll factor. It
generates an if-then-else sequence to jump into a factor -1
times unrolled loop body, like

    extraiters = tripcount % loopfactor
    if (extraiters == 0) jump Loop:
    if (extraiters == loopfactor) jump L1
    if (extraiters == loopfactor-1) jump L2
    ...
    L1:  LoopBody;
    L2:  LoopBody;
    ...
    if tripcount < loopfactor jump End
    Loop:
    ...
    End:

It means if the unroll factor is 4, the loop body will be 7
times unrolled, 3 are in loop prologue, and 4 are in the loop.
This commit is to use a loop to execute the extra iterations
in prologue, like

        extraiters = tripcount % loopfactor
        if (extraiters == 0) jump Loop:
        else jump Prol
 Prol:  LoopBody;
        extraiters -= 1                 // Omitted if unroll factor is 2.
        if (extraiters != 0) jump Prol: // Omitted if unroll factor is 2.
        if (tripcount < loopfactor) jump End
 Loop:
 ...
 End:

Then when unroll factor is 4, the loop body will be copied by
only 5 times, 1 in the prologue loop, 4 in the original loop.
And if the unroll factor is 2, new loop won't be created, just
as the original solution.

llvm-svn: 218604

1 files changed, 4 insertions, 4 deletions

diff --git a/llvm/test/Transforms/LoopUnroll/runtime-loop1.ll b/llvm/test/Transforms/LoopUnroll/runtime-loop1.ll
index ad99b8cd9c6..5ff75e33f7f 100644
--- a/llvm/test/Transforms/LoopUnroll/runtime-loop1.ll
+++ b/llvm/test/Transforms/LoopUnroll/runtime-loop1.ll
@@ -1,11 +1,11 @@
-; RUN: opt < %s -S -loop-unroll -unroll-runtime -unroll-count=4 | FileCheck %s
+; RUN: opt < %s -S -loop-unroll -unroll-runtime -unroll-count=2 | FileCheck %s
 
 ; This tests that setting the unroll count works
 
-; CHECK: unr.cmp:
-; CHECK: for.body.unr:
+; CHECK: for.body.prol:
+; CHECK: br label %for.body.preheader.split
 ; CHECK: for.body:
-; CHECK: br i1 %exitcond.3, label %for.end.loopexit{{.*}}, label %for.body
+; CHECK: br i1 %exitcond.1, label %for.end.loopexit.unr-lcssa, label %for.body
 ; CHECK-NOT: br i1 %exitcond.4, label %for.end.loopexit{{.*}}, label %for.body
 
 define i32 @test(i32* nocapture %a, i32 %n) nounwind uwtable readonly {