MaskRay added a comment.
I find another deficiency (infinite loop) with the current approach.
Say, there is a `je 0` (0x0F 0x84 0x00 0x00 0x00 0x00) at byte 0x90.
(0x90+6)%32 == 0, so it ends on a 32-byte boundary.
MF.getMaxPrefixSize() is 4, so the size of MCMachineDependentFragment may vary
from 0 to 4.
If there are other MCMachineDependentFragment's in the section, some may shrink
while some may expand.
In some cases the following loop will not converge
bool MCAssembler::layoutOnce(MCAsmLayout &Layout) {
++stats::RelaxationSteps;
bool WasRelaxed = false;
for (iterator it = begin(), ie = end(); it != ie; ++it) {
MCSection &Sec = *it;
while (layoutSectionOnce(Layout, Sec)) ///
WasRelaxed = true;
}
return WasRelaxed;
}
// In MCAssembler::layoutSectionOnce,
case MCFragment::FT_MachineDependent:
RelaxedFrag =
relaxMachineDependent(Layout, *cast<MCMachineDependentFragment>(I));
break;
To give a concrete example, `clang++ -fsanitize=memory
compiler-rt/test/msan/cxa_atexit.cpp -mbranches-within-32B-boundaries` does not
converge. You may also try dtor-*.cpp in that directory.
A simple iterative algorithm is not guaranteed to converge. We probably can
solve the layout problem with a dynamic programming algorithm:
f[i][j] = the minimum cost that layouts the first i instructions with j extra
bytes (via NOPs or prefixes)
or
g[i][j] = the minimum inserted bytes that layouts the first i instructions with
cost j
I am not clear which one is better. A simple greedy approach is to set an upper
limit on the number of iterations when the section contains at least one
MCMachineDependentFragment, i.e.
bool HasMCMachineDependentFragment = false;
int count = 5; // arbitrary. Please find an appropriate value.
while (layoutSectionOnce(Layout, Sec, HasMCMachineDependentFragment) && count
> 0) {
WasRelaxed = true;
if (HasMCMachineDependentFragment)
count--;
}
CHANGES SINCE LAST ACTION
https://reviews.llvm.org/D70157/new/
https://reviews.llvm.org/D70157
_______________________________________________
cfe-commits mailing list
cfe-commits@lists.llvm.org
https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
