Add llvm.licm.disable metadata

For some targets the LICM pass can result in sub-optimal code in some
cases where it would be better not to run the pass, but it isn't
always possible to suppress the transformations heuristically.

Where the front-end has insight into such cases it is beneficial
to attach loop metadata to disable the pass - this change adds the
llvm.licm.disable metadata to enable that.

Differential Revision: https://reviews.llvm.org/D64557

llvm-svn: 368296

Author: Tim Corringham

llvm/docs/LangRef.rst

@@ -5381,7 +5381,7 @@ suggests an unroll factor to the loop unroller:
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
 This metadata disables all optional loop transformations unless
-explicitly instructed using other transformation metdata such as
+explicitly instructed using other transformation metadata such as
 ``llvm.loop.unroll.enable``. That is, no heuristic will try to determine
 whether a transformation is profitable. The purpose is to avoid that the
 loop is transformed to a different loop before an explicitly requested
@@ -5720,10 +5720,24 @@ the non-distributed fallback version will have. See
 '``llvm.loop.distribute.followup_all``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 
-Thes attributes in this metdata is added to all followup loops of the
+The attributes in this metadata is added to all followup loops of the
 loop distribution pass. See
 :ref:`Transformation Metadata <transformation-metadata>` for details.
 
+'``llvm.licm.disable``' Metadata
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+This metadata indicates that loop-invariant code motion (LICM) should not be
+performed on this loop. The metadata has a single operand which is the string
+``llvm.licm.disable``. For example:
+
+.. code-block:: llvm
+
+   !0 = !{!"llvm.licm.disable"}
+
+Note that although it operates per loop it isn't given the llvm.loop prefix
+as it is not affected by the ``llvm.loop.disable_nonforced`` metadata.
+
 '``llvm.access.group``' Metadata
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

llvm/include/llvm/Transforms/Utils/LoopUtils.h

@@ -215,6 +215,9 @@ makeFollowupLoopID(MDNode *OrigLoopID, ArrayRef<StringRef> FollowupAttrs,
 /// Look for the loop attribute that disables all transformation heuristic.
 bool hasDisableAllTransformsHint(const Loop *L);
 
+/// Look for the loop attribute that disables the LICM transformation heuristics.
+bool hasDisableLICMTransformsHint(const Loop *L);
+
 /// The mode sets how eager a transformation should be applied.
 enum TransformationMode {
   /// The pass can use heuristics to determine whether a transformation should

llvm/lib/Transforms/Scalar/LICM.cpp

@@ -330,6 +330,12 @@ bool LoopInvariantCodeMotion::runOnLoop(
 
   assert(L->isLCSSAForm(*DT) && "Loop is not in LCSSA form.");
 
+  // If this loop has metadata indicating that LICM is not to be performed then
+  // just exit.
+  if (hasDisableLICMTransformsHint(L)) {
+    return false;
+  }
+
   std::unique_ptr<AliasSetTracker> CurAST;
   std::unique_ptr<MemorySSAUpdater> MSSAU;
   bool NoOfMemAccTooLarge = false;

llvm/lib/Transforms/Utils/LoopUtils.cpp

@@ -45,6 +45,7 @@ using namespace llvm::PatternMatch;
 #define DEBUG_TYPE "loop-utils"
 
 static const char *LLVMLoopDisableNonforced = "llvm.loop.disable_nonforced";
+static const char *LLVMLoopDisableLICM = "llvm.licm.disable";
 
 bool llvm::formDedicatedExitBlocks(Loop *L, DominatorTree *DT, LoopInfo *LI,
                                    MemorySSAUpdater *MSSAU,
@@ -379,6 +380,10 @@ bool llvm::hasDisableAllTransformsHint(const Loop *L) {
   return getBooleanLoopAttribute(L, LLVMLoopDisableNonforced);
 }
 
+bool llvm::hasDisableLICMTransformsHint(const Loop *L) {
+  return getBooleanLoopAttribute(L, LLVMLoopDisableLICM);
+}
+
 TransformationMode llvm::hasUnrollTransformation(Loop *L) {
   if (getBooleanLoopAttribute(L, "llvm.loop.unroll.disable"))
     return TM_SuppressedByUser;

llvm/test/Transforms/LICM/pragma-licm-disable.ll

@@ -0,0 +1,33 @@
+; RUN: opt < %s -S -basicaa -licm | FileCheck %s
+
+; Check that the LICM pass does not operate on a loop which has the
+; llvm.licm.disable metadata.
+; CHECK-LABEL: @licm_disable
+; CHECK: entry:
+; CHECK-NOT: load
+; CHECK: do.body:
+; CHECK: load i64, i64* bitcast (i32** @in to i64*)
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-pc-windows-msvc"
+
+@in = internal unnamed_addr global i32* null, align 8
+@out = internal unnamed_addr global i32* null, align 8
+
+define void @licm_disable(i32 %N) {
+entry:
+  br label %do.body
+
+do.body:                                          ; preds = %entry
+  %i.0 = phi i32 [ 0, %entry ], [ %inc, %do.body ]
+  %v1 = load i64, i64* bitcast (i32** @in to i64*), align 8
+  store i64 %v1, i64* bitcast (i32** @out to i64*), align 8
+  %inc = add nsw i32 %i.0, 1
+  %cmp = icmp slt i32 %inc, %N
+  br i1 %cmp, label %do.body, label %do.end, !llvm.loop !1
+
+do.end:                                           ; preds = %do.body
+  ret void
+}
+!1 = !{!1, !2}
+!2 = !{!"llvm.licm.disable"}