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On 4/14/11 6:34 PM, Kodakara, Sreekumar V wrote:
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<p class="MsoNormal">Hi All,<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I have a question on ConstantExpressions
and llvm intrinsic memcpy/memset/memmove. I am using llvm-2.8
release. In one of the C programs that I am compiling using
clang frontend, the call to memcpy instrinsic looks like the
following<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">call void @llvm.memcpy.p0i8.p0i8.i64(i8*
%tmp2, i8* bitcast (%struct.ta* @tret to i8*), i64 4, i32 4,
i1 false), !dbg !19<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">The second argument to memcpy is of type
“struct ta” and global variable “tret” is defined as follows
in the code.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">struct ta {<o:p></o:p></p>
<p class="MsoNormal"> int a;<o:p></o:p></p>
<p class="MsoNormal">};<o:p></o:p></p>
<p class="MsoNormal">struct ta tret;<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Since memcpy takes i8* as its argument, the
struct.ta* is bitcasted to i8* in the call to llvm.memcpy. For
a pass that I am working on, I would like to know the original
type of this pointer. In this example, I would like to know
that the second argument is a pointer to type struct.ta, and
hence the size of memory allocated to that pointer is 4 bytes.
</p>
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<br>
You should be able to use the stripPointerCasts() method of
llvm::Value * to strip off the bitcast.<br>
<br>
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<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I understand that the second argument to
memcpy intrinsic namely i8* bitcast (%struct.ta* @t1 to i8*)
is a ConstantExpression (CE). When I dump
CE->getArgument(0), I get the following. <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">@t1 = global %struct.ta zeroinitializer,
align 4<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">But from here, I am not able to find the
relevant class/API to extract the type of
CE->getArgument(0) and hence the size of it. When I tried
CE->getArgument(0)->getType(), I am getting the type as
Pointer and hence the size to be 8 bytes. </p>
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<br>
This is because all global variables in LLVM are pointers to the
actual data in memory. What you need to do is:<br>
<br>
if (GlobalVariable * GV =
dyn_cast<GlobalVariable>(CE->getArgument(0)) {<br>
Type * T = GV->getType()->getContainedType();<br>
}<br>
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<p class="MsoNormal">I would like to get the type as StructTy
and size to be 4 bytes. I searched through the llvm code <o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Any help is appreciated. </p>
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<br>
-- John T.<br>
<br>
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<p class="MsoNormal"><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks<o:p></o:p></p>
<p class="MsoNormal">Sreekumar<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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