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
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@368296
91177308-0d34-0410-b5e6-
96231b3b80d8
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
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
'``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
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
/// 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
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;
#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,
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;
--- /dev/null
+; 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"}