[cfe-dev] Question about ASTContext::getPromotedIntegerType() function

[JinGu Kang]

Hi Eli,

I appreciate your comment. It's really helpful to understand the code.

How do you feel about generating "sdiv" instruction on previous example?

Thanks,
JinGu Kang

On 16/07/2013 17:41, Eli Friedman wrote:
> On Tue, Jul 16, 2013 at 4:01 AM, JinGu Kang <jingu at codeplay.com> wrote:
>> Hi all,
>>
>> I have a question about ASTContext::getPromotedIntegerType() function.
>>
>> I have tested a code as following:
>>
>> source code:
>> int main()
>> {
>>    volatile unsigned short x;
>>    volatile unsigned short y;
>>    volatile unsigned short result;
>>
>>    x = 8;
>>    y = 2;
>>
>>    result = x / y;
>>
>>    return 0;
>> }
>>
>> Generated IR from clang:
>>    6 define i32 @main() #0 {
>>    7 entry:
>>    8   %retval = alloca i32, align 4
>>    9   %x = alloca i16, align 2
>>   10   %y = alloca i16, align 2
>>   11   %result = alloca i16, align 2
>>   12   store i32 0, i32* %retval
>>   13   store volatile i16 8, i16* %x, align 2
>>   14   store volatile i16 2, i16* %y, align 2
>>   15   %0 = load volatile i16* %x, align 2
>>   16   %conv = zext i16 %0 to i32
>>   17   %1 = load volatile i16* %y, align 2
>>   18   %conv1 = zext i16 %1 to i32
>>   19   %div = sdiv i32 %conv, %conv1
>>   20   %conv2 = trunc i32 %div to i16
>>   21   store volatile i16 %conv2, i16* %result, align 2
>>   22   ret i32 0
>>   23 }
>>
>> I expected "udiv" instruction on line 19 but there was "sdiv" instruction
>> because of "ASTContext::getPromotedIntegerType()" function promotes
>> "unsigned short" to "int".
>>
>> Could someone explain why this function returns "IntTy" when
>> "PromotableSize" is not same with "IntSize"?
> Because that's what the standard says.  C11 6.3.1.1: "If an int can
> represent all values of the original type (as restricted by the width,
> for a bit-field), the value is converted to an int; otherwise, it is
> converted to an unsigned int."
>
> -Eli

-- 
JinGu Kang
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