-Wframe-larger-than no longer shows information about spills/variables

[nathanchance]

While investigating a recent -Wframe-larger-than warning, I noticed that on recent tip of tree LLVM builds, we no longer see information about the spills/variables that were added by llvm/llvm-project@2e1e2f5.

LLVM 16.0.6:

fs/smb/client/sess.c:160:5: warning: stack frame size (1152) exceeds limit (1024) in 'cifs_try_adding_channels' [-Wframe-larger-than]
int cifs_try_adding_channels(struct cifs_sb_info *cifs_sb, struct cifs_ses *ses)
    ^
412/1152 (35.76%) spills, 740/1152 (64.24%) variables
1 warning generated.

LLVM 17.0.0:

fs/smb/client/sess.c:160:5: warning: stack frame size (1152) exceeds limit (1024) in 'cifs_try_adding_channels' [-Wframe-larger-than]
  160 | int cifs_try_adding_channels(struct cifs_sb_info *cifs_sb, struct cifs_ses *ses)
      |     ^
1 warning generated.

It looks like llvm/llvm-project@945f6e6 removed this, which basically defeats the purpose of llvm/llvm-project@2e1e2f5 from what I can tell... While we can still get this information from KCFLAGS=-Rpass-analysis=stack-frame-layout, I think it is helpful for other non-compiler developers to immediately see where the high stack usage comes from, as spills typically indicate something up with inlining or other internal compiler decisions whereas variables are more indicative of something wrong with the code.

[nathanchance]

There was some post-commit discussion on Phabricator: https://reviews.llvm.org/rG945f6e65be0d20b3446e7c1537c64151de618af4

[nickdesaulniers]

cc @ilovepi

[nickdesaulniers]

This may be working as intended.

Though I will note from this example there are cases where -Rpass-analysis=stack-frame-layout is not able to inform what variable is in what stack slot. In contrast, frame-larger-than can spot this info from the debug info.

[nickdesaulniers]

I get 1 variable info for -Rpass-analysis=stack-frame-layout when using --target=arm-linux-gnueabihf and many unknown stack slots...example:

...
Offset: [SP-40], Type: Variable, Align: 4, Size: 4
Offset: [SP-44], Type: Protector, Align: 4, Size: 4
Offset: [SP-56], Type: Variable, Align: 8, Size: 8
    res @ ./drivers/gpu/drm/amd/amdgpu/../display/dc/inc/bw_fixed.h:128
Offset: [SP-64], Type: Variable, Align: 8, Size: 8
Offset: [SP-72], Type: Variable, Align: 8, Size: 8
Offset: [SP-80], Type: Variable, Align: 8, Size: 8
Offset: [SP-84], Type: Spill, Align: 4, Size: 4
Offset: [SP-88], Type: Spill, Align: 4, Size: 4
Offset: [SP-92], Type: Spill, Align: 4, Size: 4
Offset: [SP-96], Type: Spill, Align: 4, Size: 4
Offset: [SP-100], Type: Spill, Align: 4, Size: 4
Offset: [SP-104], Type: Spill, Align: 4, Size: 4
Offset: [SP-108], Type: Spill, Align: 4, Size: 4
Offset: [SP-112], Type: Spill, Align: 4, Size: 4
Offset: [SP-116], Type: Spill, Align: 4, Size: 4
Offset: [SP-120], Type: Spill, Align: 4, Size: 4
...