[LLVMdev] [PATCH] LangRef: va_arg doesn't work on X86_64; update example (original) (raw)
Ramkumar Ramachandra artagnon at gmail.com
Tue Oct 28 18:32:12 PDT 2014
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Provide a full-fledged example of working variable arguments on X86_64, since it's easily the most popular platform.
Cc: Reid Kleckner <rnk at google.com> Signed-off-by: Ramkumar Ramachandra <artagnon at gmail.com>
docs/LangRef.rst | 44 ++++++++++++++++++++++++++++++-------------- 1 file changed, 30 insertions(+), 14 deletions(-)
diff --git a/docs/LangRef.rst b/docs/LangRef.rst
index ddef803..f429062 100644
--- a/docs/LangRef.rst
+++ b/docs/LangRef.rst
@@ -6695,7 +6695,8 @@ Overview:
The 'va_arg' instruction is used to access arguments passed through
the "variable argument" area of a function call. It is used to implement
-the va_arg macro in C.
+the va_arg macro in C. Note that va_arg does not work on
+platforms that have a non-builtin va_list type, like x86_64.
Arguments:
""""""""""
@@ -6879,29 +6880,44 @@ value type "va_list". The LLVM assembly language reference manual
does not define what this type is, so all transformations should be
prepared to handle these functions regardless of the type used.
-This example shows how the :ref:va_arg <i_va_arg> instruction and the
-variable argument handling intrinsic functions are used.
+This example shows how the variable argument handling is done on
+x86_64. The complexity arises from the fact that va_arg does not
+work on this type of va_list.
.. code-block:: llvm
+ %struct.__va_list_tag = type { i32, i32, i8*, i8* }
+
define i32 @test(i32 %X, ...) {
; Initialize variable argument processing
- %ap = alloca i8*
- %ap2 = bitcast i8** %ap to i8*
+ %ap = alloca %struct.__va_list_tag
+ %ap2 = bitcast %struct.__va_list_tag* %ap to i8*
+ %retptr = alloca i32
call void @llvm.va_start(i8* %ap2)
; Read a single integer argument
- %tmp = va_arg i8** %ap, i32
; Demonstrate usage of llvm.va_copy and llvm.va_end%aq = alloca i8*%aq2 = bitcast i8** %aq to i8*call void @llvm.va_copy(i8* %aq2, i8* %ap2)call void @llvm.va_end(i8* %aq2)
%idxptr = getelementptr inbounds %struct.__va_list_tag* %ap, i32 0, i32 0%idx = load i32* %idxptr%tmp = getelementptr inbounds %struct.__va_list_tag* %ap, i32 0, i32 3%extract = load i8** %tmp%rawel = getelementptr i8* %extract, i32 %idx%elptr = bitcast i8* %rawel to i32*%newidx = add i32 %idx, 8store i32 %idx, i32* %idxptr; Store that argument in el%el = load i32* %elptrstore i32 %el, i32* %retptr%ret = load i32* %retptr- ; Stop processing of arguments. call void @llvm.va_end(i8* %ap2)
ret i32 %tmp
ret i32 %ret}
define i32 @main() {
%call = call i32 (i32, ...)* @test(i32 1, i32 3)ret i32 %call}
declare void @llvm.va_start(i8*)
-- 1.9.3 (Apple Git-50)
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