SimplifyCFG fix for volatile load/store.

Fixes rdar:13349374.

Volatile loads and stores need to be preserved even if the language
standard says they are undefined. "volatile" in this context means "get
out of the way compiler, let my platform handle it".

Additionally, this is the only way I know of with llvm to write to the
first page (when hardware allows) without dropping to assembly.

llvm-svn: 176599

2 files changed, 52 insertions, 2 deletions

diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index a63d31d5af5..a5c5c22aa5a 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -3944,11 +3944,13 @@ static bool passingValueIsAlwaysUndefined(Value *V, Instruction *I) {
 
     // Load from null is undefined.
     if (LoadInst *LI = dyn_cast<LoadInst>(Use))
-      return LI->getPointerAddressSpace() == 0;
+      if (!LI->isVolatile())
+        return LI->getPointerAddressSpace() == 0;
 
     // Store to null is undefined.
     if (StoreInst *SI = dyn_cast<StoreInst>(Use))
-      return SI->getPointerAddressSpace() == 0 && SI->getPointerOperand() == I;
+      if (!SI->isVolatile())
+        return SI->getPointerAddressSpace() == 0 && SI->getPointerOperand() == I;
   }
   return false;
 }
diff --git a/llvm/test/Transforms/SimplifyCFG/volatile-phioper.ll b/llvm/test/Transforms/SimplifyCFG/volatile-phioper.ll
new file mode 100644
index 00000000000..164898897ef
--- /dev/null
+++ b/llvm/test/Transforms/SimplifyCFG/volatile-phioper.ll
@@ -0,0 +1,48 @@
+; RUN: opt < %s -simplifycfg -S | FileCheck %s
+;
+; rdar:13349374
+;
+; SimplifyCFG should not eliminate blocks with volatile stores.
+; Essentially, volatile needs to be backdoor that tells the optimizer
+; it can no longer use language standard as an excuse. The compiler
+; needs to expose the volatile access to the platform.
+;
+; CHECK: @test
+; CHECK: entry:
+; CHECK: @Trace
+; CHECK: while.body:
+; CHECK: store volatile
+; CHECK: end:
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128-n8:16:32:64-S128"
+
+define void @test(i8** nocapture %PeiServices) #0 {
+entry:
+  %call = tail call i32 (...)* @Trace() #2
+  %tobool = icmp eq i32 %call, 0
+  br i1 %tobool, label %while.body, label %if.then
+
+if.then:                                          ; preds = %entry
+  %call1 = tail call i32 (...)* @Trace() #2
+  br label %while.body
+
+while.body:                                       ; preds = %entry, %if.then, %while.body
+  %Addr.017 = phi i8* [ %incdec.ptr, %while.body ], [ null, %if.then ], [ null, %entry ]
+  %x.016 = phi i8 [ %inc, %while.body ], [ 0, %if.then ], [ 0, %entry ]
+  %inc = add i8 %x.016, 1
+  %incdec.ptr = getelementptr inbounds i8* %Addr.017, i64 1
+  store volatile i8 %x.016, i8* %Addr.017, align 1
+  %0 = ptrtoint i8* %incdec.ptr to i64
+  %1 = trunc i64 %0 to i32
+  %cmp = icmp ult i32 %1, 4096
+  br i1 %cmp, label %while.body, label %end
+
+end:
+  ret void
+}
+declare i32 @Trace(...) #1
+
+attributes #0 = { nounwind ssp uwtable "fp-contract-model"="standard" "no-frame-pointer-elim" "no-frame-pointer-elim-non-leaf" "realign-stack" "relocation-model"="pic" "ssp-buffers-size"="8" }
+attributes #1 = { "fp-contract-model"="standard" "no-frame-pointer-elim" "no-frame-pointer-elim-non-leaf" "realign-stack" "relocation-model"="pic" "ssp-buffers-size"="8" }
+attributes #2 = { nounwind }
+
+!0 = metadata !{i32 1039}