PNaCl: Rewrite @llvm.assume calls to nothing.

@llvm.assume calls can safely be removed.

R=jfb@chromium.org
TEST= (make -C toolchain_build/out/llvm_x86_64_linux_work -j `nproc` check)
BUG=

Review URL: https://codereview.chromium.org/1029273002

Author: jfbastien

lib/Transforms/NaCl/RewriteLLVMIntrinsics.cpp

@@ -115,8 +115,10 @@ bool RewriteLLVMIntrinsics::runOnModule(Module &M) {
 
   // Remove all @llvm.prefetch intrinsics.
   ToNothing PrefetchRewriter(M, Intrinsic::prefetch);
+  ToNothing AssumeRewriter(M, Intrinsic::assume);
 
-  return visitUses(FltRoundsRewriter) | visitUses(PrefetchRewriter);
+  return visitUses(FltRoundsRewriter) | visitUses(PrefetchRewriter)
+    | visitUses(AssumeRewriter);
 }
 
 bool RewriteLLVMIntrinsics::visitUses(IntrinsicRewriter &Rewriter) {

test/Transforms/NaCl/rewrite-assume.ll

@@ -0,0 +1,35 @@
+; RUN: opt < %s -rewrite-llvm-intrinsic-calls -S | FileCheck %s
+; RUN: opt < %s -rewrite-llvm-intrinsic-calls -S | FileCheck %s -check-prefix=CLEANED
+; Test the @llvm.assume part of the RewriteLLVMIntrinsics pass
+
+declare void @llvm.assume(i1)
+
+; No declaration or definition of llvm.assume() should remain.
+; CLEANED-NOT: @llvm.assume
+
+define void @call_assume(i1 %val) {
+; CHECK: call_assume
+; CHECK-NEXT: ret void
+  call void @llvm.assume(i1 %val)
+  ret void
+}
+
+; A more complex example with a number of calls in several BBs.
+define void @multiple_calls(i1 %val) {
+; CHECK: multiple_calls
+entryblock:
+; CHECK: entryblock
+; CHECK-NEXT: br
+  call void @llvm.assume(i1 %val)
+  br i1 %val, label %exitblock, label %never
+never:
+; CHECK: never:
+; CHECK-NEXT: br
+  call void @llvm.assume(i1 %val)
+  br label %exitblock
+exitblock:
+; CHECK: exitblock:
+; CHECK-NEXT: ret void
+  call void @llvm.assume(i1 %val)
+  ret void
+}