further clarify alignment of globals, fix instcombine

to not increase the alignment of globals with an assigned
alignment and section.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@102476 91177308-0d34-0410-b5e6-96231b3b80d8

2 files changed, 25 insertions, 21 deletions

diff --git a/docs/LangRef.html b/docs/LangRef.html
index 05637ab330..02b249b48d 100644
--- a/docs/LangRef.html
+++ b/docs/LangRef.html
@@ -855,10 +855,11 @@ define i32 @main() {                                        <i>; i32()* </i>
    of 2.  If not present, or if the alignment is set to zero, the alignment of
    the global is set by the target to whatever it feels convenient.  If an
    explicit alignment is specified, the global is forced to have exactly that
-   alignment.  Targets are not allowed to over-align the global in cases where
-   it is observable: for example, overaligning a global is observable if it has
-   an assigned section and higher alignment could cause holes between
-   consequtive globals.</p>
+   alignment.  Targets and optimizers are not allowed to over-align the global
+   if the global has an assigned section.  In this case, the extra alignment
+   could be observable: for example, code could assume that the globals are
+   densely packed in their section and try to iterate over them as an array,
+   alignment padding would break this iteration.</p>
 
 <p>For example, the following defines a global in a numbered address space with
    an initializer, section, and alignment:</p>
diff --git a/lib/Transforms/InstCombine/InstCombineCalls.cpp b/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 5e98d5b475..38e7b6ec2d 100644
--- a/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -59,29 +59,32 @@ static unsigned EnforceKnownAlignment(Value *V,
       // Treat this like a bitcast.
       return EnforceKnownAlignment(U->getOperand(0), Align, PrefAlign);
     }
-    break;
+    return Align;
+  }
+  case Instruction::Alloca: {
+    AllocaInst *AI = cast<AllocaInst>(V);
+    // If there is a requested alignment and if this is an alloca, round up.
+    if (AI->getAlignment() >= PrefAlign)
+      return AI->getAlignment();
+    AI->setAlignment(PrefAlign);
+    return PrefAlign;
   }
   }
 
   if (GlobalValue *GV = dyn_cast<GlobalValue>(V)) {
     // If there is a large requested alignment and we can, bump up the alignment
     // of the global.
-    if (!GV->isDeclaration()) {
-      if (GV->getAlignment() >= PrefAlign)
-        Align = GV->getAlignment();
-      else {
-        GV->setAlignment(PrefAlign);
-        Align = PrefAlign;
-      }
-    }
-  } else if (AllocaInst *AI = dyn_cast<AllocaInst>(V)) {
-    // If there is a requested alignment and if this is an alloca, round up.
-    if (AI->getAlignment() >= PrefAlign)
-      Align = AI->getAlignment();
-    else {
-      AI->setAlignment(PrefAlign);
-      Align = PrefAlign;
-    }
+    if (GV->isDeclaration()) return Align;
+    
+    if (GV->getAlignment() >= PrefAlign)
+      return GV->getAlignment();
+    // We can only increase the alignment of the global if it has no alignment
+    // specified or if it is not assigned a section.  If it is assigned a
+    // section, the global could be densely packed with other objects in the
+    // section, increasing the alignment could cause padding issues.
+    if (!GV->hasSection() || GV->getAlignment() == 0)
+      GV->setAlignment(PrefAlign);
+    return GV->getAlignment();
   }
 
   return Align;