From 4f64f1ba3c540cddbcf3f7f5e5b8a2fd9ebbb416 Mon Sep 17 00:00:00 2001 From: Tim Corringham Date: Thu, 8 Aug 2019 13:46:17 +0000 Subject: [PATCH] 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 --- llvm/docs/LangRef.rst | 18 +++++++++++-- llvm/include/llvm/Transforms/Utils/LoopUtils.h | 3 +++ llvm/lib/Transforms/Scalar/LICM.cpp | 6 +++++ llvm/lib/Transforms/Utils/LoopUtils.cpp | 5 ++++ llvm/test/Transforms/LICM/pragma-licm-disable.ll | 33 ++++++++++++++++++++++++ 5 files changed, 63 insertions(+), 2 deletions(-) create mode 100644 llvm/test/Transforms/LICM/pragma-licm-disable.ll diff --git a/llvm/docs/LangRef.rst b/llvm/docs/LangRef.rst index b6979e5..a169589 100644 --- a/llvm/docs/LangRef.rst +++ b/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 ` 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 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ diff --git a/llvm/include/llvm/Transforms/Utils/LoopUtils.h b/llvm/include/llvm/Transforms/Utils/LoopUtils.h index 533cde1..d32f087 100644 --- a/llvm/include/llvm/Transforms/Utils/LoopUtils.h +++ b/llvm/include/llvm/Transforms/Utils/LoopUtils.h @@ -215,6 +215,9 @@ makeFollowupLoopID(MDNode *OrigLoopID, ArrayRef 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 diff --git a/llvm/lib/Transforms/Scalar/LICM.cpp b/llvm/lib/Transforms/Scalar/LICM.cpp index d9dda4c..e018eb6 100644 --- a/llvm/lib/Transforms/Scalar/LICM.cpp +++ b/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 CurAST; std::unique_ptr MSSAU; bool NoOfMemAccTooLarge = false; diff --git a/llvm/lib/Transforms/Utils/LoopUtils.cpp b/llvm/lib/Transforms/Utils/LoopUtils.cpp index ce296b9..4c6c2ed 100644 --- a/llvm/lib/Transforms/Utils/LoopUtils.cpp +++ b/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; diff --git a/llvm/test/Transforms/LICM/pragma-licm-disable.ll b/llvm/test/Transforms/LICM/pragma-licm-disable.ll new file mode 100644 index 0000000..cba1099 --- /dev/null +++ b/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"} -- 2.7.4