[LoopUnroll] Honor '#pragma unroll' even with -fno-unroll-loops.

When using clang with `-fno-unroll-loops` (implicitly added with `-O1`),
the LoopUnrollPass is not not added to the (legacy) pass pipeline. This
also means that it will not process any loop metadata such as
llvm.loop.unroll.enable (which is generated by #pragma unroll or
WarnMissedTransformationsPass emits a warning that a forced
transformation has not been applied (see
https://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20181210/610833.html).
Such explicit transformations should take precedence over disabling
heuristics.

This patch unconditionally adds LoopUnrollPass to the optimizing
pipeline (that is, it is still not added with `-O0`), but passes a flag
indicating whether automatic unrolling is dis-/enabled. This is the same
approach as LoopVectorize uses.

The new pass manager's pipeline builder has no option to disable
unrolling, hence the problem does not apply.

Differential Revision: https://reviews.llvm.org/D55716

llvm-svn: 349509

1 files changed, 15 insertions, 15 deletions

diff --git a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
index e625433a8e4..268939e8247 100644
--- a/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
+++ b/llvm/lib/Transforms/IPO/PassManagerBuilder.cpp
@@ -378,8 +378,8 @@ void PassManagerBuilder::addFunctionSimplificationPasses(
   if (EnableLoopInterchange)
     MPM.add(createLoopInterchangePass()); // Interchange loops
 
-  if (!DisableUnrollLoops)
-    MPM.add(createSimpleLoopUnrollPass(OptLevel));    // Unroll small loops
+  MPM.add(createSimpleLoopUnrollPass(OptLevel,
+                                     DisableUnrollLoops)); // Unroll small loops
   addExtensionsToPM(EP_LoopOptimizerEnd, MPM);
   // This ends the loop pass pipelines.
 
@@ -682,16 +682,17 @@ void PassManagerBuilder::populateModulePassManager(
   addExtensionsToPM(EP_Peephole, MPM);
   addInstructionCombiningPass(MPM);
 
-  if (!DisableUnrollLoops) {
-    if (EnableUnrollAndJam) {
-      // Unroll and Jam. We do this before unroll but need to be in a separate
-      // loop pass manager in order for the outer loop to be processed by
-      // unroll and jam before the inner loop is unrolled.
-      MPM.add(createLoopUnrollAndJamPass(OptLevel));
-    }
+  if (EnableUnrollAndJam && !DisableUnrollLoops) {
+    // Unroll and Jam. We do this before unroll but need to be in a separate
+    // loop pass manager in order for the outer loop to be processed by
+    // unroll and jam before the inner loop is unrolled.
+    MPM.add(createLoopUnrollAndJamPass(OptLevel));
+  }
 
-    MPM.add(createLoopUnrollPass(OptLevel));    // Unroll small loops
+  MPM.add(createLoopUnrollPass(OptLevel,
+                               DisableUnrollLoops)); // Unroll small loops
 
+  if (!DisableUnrollLoops) {
     // LoopUnroll may generate some redundency to cleanup.
     addInstructionCombiningPass(MPM);
 
@@ -700,7 +701,7 @@ void PassManagerBuilder::populateModulePassManager(
     // outer loop. LICM pass can help to promote the runtime check out if the
     // checked value is loop invariant.
     MPM.add(createLICMPass());
- }
+  }
 
   MPM.add(createWarnMissedTransformationsPass());
 
@@ -872,12 +873,11 @@ void PassManagerBuilder::addLTOOptimizationPasses(legacy::PassManagerBase &PM) {
   if (EnableLoopInterchange)
     PM.add(createLoopInterchangePass());
 
-  if (!DisableUnrollLoops)
-    PM.add(createSimpleLoopUnrollPass(OptLevel));   // Unroll small loops
+  PM.add(createSimpleLoopUnrollPass(OptLevel,
+                                    DisableUnrollLoops)); // Unroll small loops
   PM.add(createLoopVectorizePass(true, LoopVectorize));
   // The vectorizer may have significantly shortened a loop body; unroll again.
-  if (!DisableUnrollLoops)
-    PM.add(createLoopUnrollPass(OptLevel));
+  PM.add(createLoopUnrollPass(OptLevel, DisableUnrollLoops));
 
   PM.add(createWarnMissedTransformationsPass());