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further clarify alignment of globals, fix instcombine

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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
This commit is contained in:
Chris Lattner 2010-04-28 00:31:12 +00:00
parent ce99fa9e26
commit 2d4b8ee1d9
2 changed files with 25 additions and 21 deletions
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9
docs/LangRef.html
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@ -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>

37
lib/Transforms/InstCombine/InstCombineCalls.cpp
View File

@ -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;