On Wed, Feb 20, 2019 at 1:40 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > On Wed, Feb 20, 2019 at 10:13 PM Arnd Bergmann <arnd@xxxxxxxx> wrote: > > > > In the example in https://bugs.llvm.org/show_bug.cgi?id=38809#c12 > > (https://godbolt.org/z/ylsGSQ) there is no inlining, yet clang uses > > over ten times as much stack space as gcc, for reasons I still > > can't explain. My assumption right now is that the underlying bug > > causes most of the problems with excessive stack usage in > > allmodconfig kernels. > > Here is an even more minimal example: > > struct s { int i[5]; } f(void); > void g(void) { f(); f();} On this example I can see some stupidity that clang/asan is doing. Let me try fixing it and see if it helps bigger cases. Thanks for reducing the case! This is the input we get in the asan instrumentation: ; Function Attrs: noinline nounwind optnone sanitize_address uwtable define dso_local void @g() #0 { entry: %tmp = alloca %struct.s, align 4 <<<<<<<<<<<<<<<<<<<<<<< %tmp1 = alloca %struct.s, align 4 %0 = bitcast %struct.s* %tmp to i8* call void @llvm.lifetime.start.p0i8(i64 20, i8* %0) #3 call void @f(%struct.s* sret %tmp) %1 = bitcast %struct.s* %tmp to i8* <<<<<<<<<<<<<<<<<<<<< call void @llvm.lifetime.end.p0i8(i64 20, i8* %1) #3 %2 = bitcast %struct.s* %tmp1 to i8* call void @llvm.lifetime.start.p0i8(i64 20, i8* %2) #3 <<<<<<<<<<<<< call void @f(%struct.s* sret %tmp1) %3 = bitcast %struct.s* %tmp1 to i8* call void @llvm.lifetime.end.p0i8(i64 20, i8* %3) #3 ret void } the stack variables are not *really* used, but since they are "used" inside the lifetime markers they are not eliminated by asan, and so asan instruments them, after which no one can remove them any more... > > https://godbolt.org/z/d_KWkh > > It's clear that clang does /something/ here when asan-stack=1 is > set, but I fail to see what it is, or why that is necessary. > > The output of clang with asan-stack=0 is the expected > code, and basically identical to what gcc produces with or > without asan-stack. > > Arnd
