Commit e3d8ee35e4 ("reland "[DebugInfo] Support to emit debugInfo
for extern variables"") added support to emit debugInfo for
extern variables if requested by the target. Currently, only
BPF target enables this feature by default.
As BPF ecosystem grows, callback function started to get
support, e.g., recently bpf_for_each_map_elem() is introduced
(https://lwn.net/Articles/846504/) with a callback function as an
argument. In the future we may have something like below as
a demonstration of use case :
extern int do_work(int);
long bpf_helper(void *callback_fn, void *callback_ctx, ...);
long prog_main() {
struct { ... } ctx = { ... };
return bpf_helper(&do_work, &ctx, ...);
}
Basically bpf helper may have a callback function and the
callback function is defined in another file or in the kernel.
In this case, we would like to know the debuginfo types for
do_work(), so the verifier can proper verify the safety of
bpf_helper() call.
For the following example,
extern int do_work(int);
long bpf_helper(void *callback_fn);
long prog() {
return bpf_helper(&do_work);
}
Currently, there is no debuginfo generated for extern function do_work().
In the IR, we have,
...
define dso_local i64 @prog() local_unnamed_addr #0 !dbg !7 {
entry:
%call = tail call i64 @bpf_helper(i8* bitcast (i32 (i32)* @do_work to i8*)) #2, !dbg !11
ret i64 %call, !dbg !12
}
...
declare dso_local i32 @do_work(i32) #1
...
This patch added support for the above callback function use case, and
the generated IR looks like below:
...
declare !dbg !17 dso_local i32 @do_work(i32) #1
...
!17 = !DISubprogram(name: "do_work", scope: !1, file: !1, line: 1, type: !18, flags: DIFlagPrototyped, spFlags: DISPFlagOptimized, retainedNodes: !2)
!18 = !DISubroutineType(types: !19)
!19 = !{!20, !20}
!20 = !DIBasicType(name: "int", size: 32, encoding: DW_ATE_signed)
The TargetInfo.allowDebugInfoForExternalVar is renamed to
TargetInfo.allowDebugInfoForExternalRef as now it guards
both extern variable and extern function debuginfo generation.
Differential Revision: https://reviews.llvm.org/D100567
IRgen optimization opportunities.
//===---------------------------------------------------------------------===//
The common pattern of
--
short x; // or char, etc
(x == 10)
--
generates an zext/sext of x which can easily be avoided.
//===---------------------------------------------------------------------===//
Bitfields accesses can be shifted to simplify masking and sign
extension. For example, if the bitfield width is 8 and it is
appropriately aligned then is is a lot shorter to just load the char
directly.
//===---------------------------------------------------------------------===//
It may be worth avoiding creation of alloca's for formal arguments
for the common situation where the argument is never written to or has
its address taken. The idea would be to begin generating code by using
the argument directly and if its address is taken or it is stored to
then generate the alloca and patch up the existing code.
In theory, the same optimization could be a win for block local
variables as long as the declaration dominates all statements in the
block.
NOTE: The main case we care about this for is for -O0 -g compile time
performance, and in that scenario we will need to emit the alloca
anyway currently to emit proper debug info. So this is blocked by
being able to emit debug information which refers to an LLVM
temporary, not an alloca.
//===---------------------------------------------------------------------===//
We should try and avoid generating basic blocks which only contain
jumps. At -O0, this penalizes us all the way from IRgen (malloc &
instruction overhead), all the way down through code generation and
assembly time.
On 176.gcc:expr.ll, it looks like over 12% of basic blocks are just
direct branches!
//===---------------------------------------------------------------------===//
a2a3ca8d97