[llvm-dev] non-temporal loads and stores

[Manuel Jacob via llvm-dev]

Hi Will,

Declaring a global "char* p;" allocates memory for a pointer to a char.  
Using the identifier "p" in an expression loads a pointer from this 
memory location.  The loaded pointer is then passed to 
__builtin_nontemporal_load() / __builtin_nontemporal_store() in your 
examples.

A simpler example:

char *p;
char *test() {
     return p;
}

is compiled to:

@p = common global i8* null, align 8
define i8* @test() {
   %1 = load i8*, i8** @p, align 8
   ret i8* %1
}

-Manuel


On 2016-05-12 11:27, Will via llvm-dev wrote:
> I am trying to understand how the __builtin_nontemporal_load() and
> __builtin_nontemporal_store() are supposed to work.
> 
> When called with a global pointer they seem to do a double-indirection 
> thingy.
> 
> Here is a simple test program:
> 
> char foo(char* p) {
>     return __builtin_nontemporal_load(p);
> }
> 
> void bar(char* p, char a) {
>     __builtin_nontemporal_store(a, p);
> }
> 
> char* p;
> 
> char baz() {
>     return __builtin_nontemporal_load(p);
> }
> 
> void boo(char a) {
>     __builtin_nontemporal_store(a, p);
> }
> 
> And here is the LLVM IR:
> 
> define signext i8 @foo(i8* nocapture readonly %p) #0 {
> entry:
>   %0 = load i8, i8* %p, align 1, !tbaa !1, !nontemporal !4
>   ret i8 %0
> }
> 
> define void @bar(i8* nocapture %p, i8 signext %a) #1 {
> entry:
>   store i8 %a, i8* %p, align 1, !tbaa !1, !nontemporal !4
>   ret void
> }
> 
> @p = common global i8* null, align 8
> 
> define signext i8 @baz() #0 {
> entry:
>   %0 = load i8*, i8** @p, align 8, !tbaa !5
>   %1 = load i8, i8* %0, align 1, !tbaa !1, !nontemporal !4
>   ret i8 %1
> }
> 
> define void @boo(i8 signext %a) #1 {
> entry:
>   %0 = load i8*, i8** @p, align 8, !tbaa !5
>   store i8 %a, i8* %0, align 1, !tbaa !1, !nontemporal !4
>   ret void
> }
> 
> Why do baz() and boo() load indirectly?
> 
> Apologies if I'm missing something blindingly obvious here ;)
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