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John,<br>
<br>
Thanks for the reply.<br>
I agree with your comments that the "Memory" LLVM Spec refers to
doesn't include stack.<br>
<br>
Let me leverage a bit further:<br>
<br>
If I need to work on high-level IRs (not machine dependent, not in
the code-gen stage), is it reasonable to assume that<br>
ALL LLVM IRs that have a result field will have potential to write
stack?<br>
<br>
<br>
E.g.<br>
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<pre> <result> = add <ty> <op1>, <op2> <i>; yields {ty}:result</i></pre>
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<pre> br i1 <cond>, label <iftrue>, label <iffalse>
br label <dest> <i>; Unconditional branch</i></pre>
</span></span><br>
ADD can (potential) write stack to store its result, while BR will
NEVER write stack because its doesn't have a result.<br>
<br>
<br>
Thank you<br>
<br>
Chuck<br>
<br>
<br>
<br>
<br>
On 1/21/2011 5:33 PM, John Criswell wrote:
<blockquote cite="mid:4D3A09D7.4090603@illinois.edu" type="cite">
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On 1/21/11 2:50 PM, Chuck Zhao wrote:
<blockquote cite="mid:4D39F197.6090306@eecg.toronto.edu"
type="cite">
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I need to figure out all LLVM Instructions that may write to
memory.<br>
<br>
In <a moz-do-not-send="true"
href="http://llvm.org/docs/tutorial/OCamlLangImpl7.html">http://llvm.org/docs/tutorial/OCamlLangImpl7.html</a>,
it mentions that<br>
"<span class="Apple-style-span" style="border-collapse:
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widows: 2; word-spacing: 0px; font-size: medium;"><span
class="Apple-style-span" style="text-align: left;">In LLVM,
all memory accesses are explicit with load/store
instructions, and it is carefully designed not to have (or
need) an "address-of" operator.</span></span>"<br>
<br>
I take this as "StoreInst is the only one that writes to
memory". <br>
</blockquote>
<br>
There are intrinsic functions which write to memory also, such as
memcpy.<br>
<blockquote cite="mid:4D39F197.6090306@eecg.toronto.edu"
type="cite"> <br>
However, this doesn't seem to be enough.<br>
</blockquote>
<br>
Your observation is correct. Strictly speaking, any instruction
can write to memory after code generation because it may access a
stack spill slot or a function parameter which the ABI places on
the stack.<br>
<br>
When the Language Reference Manual talks about writing to memory,
it is talking about writing to memory that is visible at the LLVM
IR level. The stack frame is invisible at the LLVM IR level. Put
another way, "memory" is a set of memory locations which can be
explicitly accessed by LLVM load and store instructions and are
not in SSA form; it is not all of the memory within the computer.<br>
<br>
If you're interested in finding instructions that write to RAM
(including writes to stack spill slots), it may be better to work
on Machine Instructions within the code generator framework.<br>
<br>
-- John T.<br>
<br>
<br>
<blockquote cite="mid:4D39F197.6090306@eecg.toronto.edu"
type="cite"> <br>
Consider: <br>
...<br>
int a, b, d;<br>
d = a + b;<br>
...<br>
<br>
The above code is turned into LLVM IR:<br>
<pre wrap=""> %0 = load i32* @a, align 4
%1 = load i32* @b, align 4
%2 = add nsw i32 %1, %0
store i32 %2, i32* @d, align 4
Is it possible that temps such as %0, %1 and/or %2 will NOT being register allocated later in the compilation stage, and thus left in memory?
The above code, when converted back to C level, looks like this:
...
unsigned int llvm_cbe_tmp__6;
unsigned int llvm_cbe_tmp__7;
unsigned int llvm_cbe_tmp__8;
unsigned int llvm_cbe_tmp__9;
llvm_cbe_tmp__6 = *(&a);
llvm_cbe_tmp__7 = *(&b);
llvm_cbe_tmp__8 = ((unsigned int )(((unsigned int )llvm_cbe_tmp__7) + ((unsigned int )llvm_cbe_tmp__6)));
*(&d) = llvm_cbe_tmp__8;
llvm_cbe_tmp__9 = /<b class="moz-txt-star"><span class="moz-txt-tag">*</span>tail<span class="moz-txt-tag">*</span></b>/ printf(((&_OC_str.array[((signed int )0u)])), llvm_cbe_tmp__8);
...
It seems the compiler-generated temps are _actually_ left on stack, and writes to them are actually writes to stack memory (via load, add, ...).
I am confused here.
Could somebody help to clarify it?
Thank you
Chuck
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</pre>
</blockquote>
<br>
</blockquote>
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