[PATCH] D72227: Add options for clang to align branches within 32B boundary

[Fangrui Song via Phabricator via cfe-commits]

MaskRay added inline comments.


================
Comment at: clang/include/clang/Driver/Options.td:2200
+          "from being across or against the boundary of specified size. "
+          "-x86-align-branch-boundary=0 doesn't align branches.">;
+def malign_branch_EQ
----------------
> -x86-align-branch-boundary=0

We should not expose llc internal option names.


================
Comment at: clang/lib/Driver/ToolChains/Clang.cpp:2045
+
+  if (Args.hasFlag(options::OPT_mbranches_within_32B_boundaries,
+                   options::OPT_mno_branches_within_32B_boundaries, false)) {
----------------
skan wrote:
> MaskRay wrote:
> > `OPT_mbranches_within_32B_boundaries` should provide default values which can be overridden by more specific options.
> Currently, `-mbranches-within-32B-boundaries` is equivalent to `-malign-branch-boundary=32 -malign-branch=fused+jcc+jmp -malign-branch-prefix-size=4
> 
> What is expected behaviour would be very confusing if specific options could override `-mbranches-within-32B-boundaries`. For example, if passed options are
> 
> ```
> -mbranches-within-32B-boundaries -malign-branch-boundary=32 -mno-branches-within-32B-boundaries
> ```
> What should the value of `-malign-branch-boundary` be?  Is it 32 or 0?
> 
> If we think `-mno-branches-within-32B-boundaries` is the negative form of `-mbranches-within-32B-boundaries` , then `-malign-branch-boundary` should be 32.
> 
> Or if we think `-mno-branches-within-32B-boundaries` wins since it appears at the end, and `-mno-branches-within-32B-boundaries` means no need to align branches, `-malign-branch-boundary` should be 0.
> 
> As long as we don't support specific options could override `-mbranches-within-32B-boundaries`, the trouble disappears  :-)
> 
> 
> -mbranches-within-32B-boundaries -malign-branch-boundary=32 -mno-branches-within-32B-boundaries

My preference is that the net effect will be: `-malign-branch-boundary=32`

```
If (Args.hasFlag(options::OPT_mbranches_within_32B_boundaries, options::OPT_mno_branches_within_32B_boundaries, false))
  boundary = 32;
if (const Arg *A = Args.getLastArg(options::OPT_malign_branch_EQ))
  boundary = ...
if (boundary)
  add -mllvm boundary
```

but I'd like to hear what others say. @jyknight @reames 


================
Comment at: clang/test/Driver/intel-align-branch.c:35
+// RUN: %clang -target x86_64-unknown-unknown -mbranches-within-32B-boundaries -mno-branches-within-32B-boundaries -### -c %s 2>&1 | FileCheck %s --check-prefix=CHECK-TOTAL3
+// CHECK-TOTAL3-NOT: "-mllvm" "-x86-align-branch-boundary=32" "-mllvm" "-x86-align-branch=fused+jcc+jmp" "-mllvm" "-x86-align-branch-prefix-size=4"
----------------
skan wrote:
> MaskRay wrote:
> > Add a `-target aarch64` test that other targets will get an error.
> > 
> > (I am lazy and usually just write `-target x86_64` (generic ELF) and omit `-unknown-known`)
> Currently, if  `-target aarch64` is passed to clang, we will get a warning 
> 
> > clang-10: warning: argument unused during compilation: '-mbranches-within-32B-boundaries' [-Wunused-command-line-argument]
> 
> Is this prompt not clear enough?
> 
You need a test on a different architecture to check that the warning is emitted as intended.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D72227/new/

https://reviews.llvm.org/D72227