riccibruno added inline comments.
================
Comment at: clang/lib/CodeGen/CGStmt.cpp:705
+}
+
void CodeGenFunction::EmitWhileStmt(const WhileStmt &S,
----------------
Tyker wrote:
> riccibruno wrote:
> > I believe that the lowering is incorrect. I applied your patch and here
> > ({F8571803}) is the IR that clang generates (obtained with `-O1 -S
> > -emit-llvm -Xclang -disable-llvm-passes -g0`) for this code:
> >
> > ```
> > bool f(bool i);
> > bool g(bool i);
> >
> > bool h1(bool i) {
> > if (i) [[likely]]
> > return f(i);
> > return g(i);
> > }
> >
> > bool h2(bool i) {
> > if (__builtin_expect(i, true))
> > return f(i);
> > return g(i);
> > }
> > ```
> >
> > In particular for the branch in `h1` we have:
> > ```
> > %tobool = trunc i8 %0 to i1
> > %expval = call i1 @llvm.expect.i1(i1 %tobool, i1 true)
> > br i1 %tobool, label %if.then, label %if.end
> > ```
> > Note that `%expval` is not used. Compare this to the branch in `h2`:
> > ```
> > %tobool = trunc i8 %0 to i1
> > %conv = zext i1 %tobool to i64
> > %expval = call i64 @llvm.expect.i64(i64 %conv, i64 1)
> > %tobool1 = icmp ne i64 %expval, 0
> > br i1 %tobool1, label %if.then, label %if.end
> > ```
> > where the extra conversions are because of the signature of
> > `__builtin_expect`.
> from reading the documentation it seemed to me that both were equivalent. but
> after further checking there aren't.
Looking at how the intrinsic is lowered in the `LowerExpectIntrinsic` pass (in
`handleBrSelExpect`), it seems that for a branch or select the following
patterns are supported:
```
// Handle non-optimized IR code like:
// %expval = call i64 @llvm.expect.i64(i64 %conv1, i64 1)
// %tobool = icmp ne i64 %expval, 0
// br i1 %tobool, label %if.then, label %if.end
//
// Or the following simpler case:
// %expval = call i1 @llvm.expect.i1(i1 %cmp, i1 1)
// br i1 %expval, label %if.then, label %if.end
```
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D59467/new/
https://reviews.llvm.org/D59467
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