https://bugs.llvm.org/show_bug.cgi?id=42551
Bug ID: 42551
Summary: excessive stack usage compiling linux amdgpu kernel
driver
Product: clang
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected], [email protected],
[email protected], [email protected]
Created attachment 22208
--> https://bugs.llvm.org/attachment.cgi?id=22208&action=edit
preprocessed and partially reduced file
One file in the linux kernel appears to trigger a failed optimization that
leads to large stack usage. Compiling a 32-bit ARM defconfig with the amdgpu
driver enabled, I get
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dce_calcs.c:2987:6: error: stack
frame size of 1344 bytes in function 'bw_calcs' [-Werror,-Wframe-larger-than=]
bool bw_calcs(struct dc_context *ctx,
^
drivers/gpu/drm/amd/amdgpu/../display/dc/calcs/dce_calcs.c:76:13: error: stack
frame size of 5328 bytes in function 'calculate_bandwidth'
[-Werror,-Wframe-larger-than=]
I managed to partially reduce the preprocessed source file to illustrate the
problem better:
https://godbolt.org/z/ZlDp0Z
The stack usage is highly target architecture dependent:
gcc arm-linux-gnueabi: 208 bytes
clang-8 arm-linux-gnueabi: 4144 bytes
clang-9 arm-linux-gnueabi: 4992 bytes
clang-9 aarch64-linux-gnu: 272 bytes
clang-9 powerpc64: 4272 bytes
clang-9 powerpc32: 4112 bytes
clang-9 s390-32: 4168 bytes
clang-9 s390-64: 4168 bytes
clang-9 sparc32: 10272 bytes
clang-9 sparc64: 432 bytes
$ clang-9 -Wframe-larger-than=10 --target=arm-linux-gnu -O2
-fno-strict-overflow -S dce-calcs-clang-noinline.i -m32 -Wno-unused-value
-Wno-logical-op-parentheses -Wno-return-type -Wno-implicit-int
dce-calcs-clang.i:275:4: warning: stack frame size of 4944 bytes in function
'calculate_bandwidth' [-Wframe-larger-than=]
calculate_bandwidth(struct bw_calcs_dceip *dceip, struct bw_calcs_vbios
*vbios, struct bw_calcs_data *data) {
^
Note that we build 32-bit kernels with -Wframe-larger-than=1024 because of the
highly limited available stack space, and using 5KB of stack is likely to
result in a kernel crash.
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