[ELF] ICF should respect section alignment

[phosek]

Summary

When performing ICF, we have to respect the alignment requirement of each section within each group.

[phosek]

Just for reference, this issue was originally reported by Rafael against gold and has recently bitten Chromium, after checking lld implementation Roland McGrath noticed that it has the same problem.

[ruiu]

LGTM

ELF/ICF.cpp

307

Remove else after return.

ELF/ICF.cpp

template <class ELFT> void ICF<ELFT>::run() {
   for (InputSection<ELFT> *S : V)
     // Set MSB on to avoid collisions with serial group IDs
     S->GroupId = getHash(S) | (uint64_t(1) << 63);
 
   // From now on, sections in V are ordered so that sections in
   // the same group are consecutive in the vector.
   std::stable_sort(V.begin(), V.end(),
                    [](InputSection<ELFT> *A, InputSection<ELFT> *B) {
+                     if (A->GroupId != B->GroupId)
                        return A->GroupId < B->GroupId;
+                     else
+                       // Within a group, put the highest alignment
+                       // requirement first, so that's the one we'll keep.
+                       return B->Alignment < A->Alignment;
                    });
 
   // Compare static contents and assign unique IDs for each static content.
   forEachGroup(V, equalsConstant);
 
   // Split groups by comparing relocations until we get a convergence.
   int Cnt = 1;
   for (;;) {

test/ELF/icf7.s

 # REQUIRES: x86
 
 # RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
 # RUN: ld.lld %t -o %t2 --icf=all --verbose | FileCheck %s
 # RUN: llvm-objdump -t %t2 | FileCheck -check-prefix=ALIGN %s
 
-# CHECK: selected .text.f1
-# CHECK:   removed .text.f2
+# CHECK: selected .text.f2
+# CHECK:   removed .text.f1
 
 # ALIGN: 0000000000011000 .text 00000000 _start
 # ALIGN: 0000000000011100 .text 00000000 f1
 
 .globl _start, f1, f2
 _start:
   ret