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

1 files changed, 33 insertions, 0 deletions

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 00000000000..cba109943e3
--- /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"}