Reland r338088, "ELF: Make --print-icf-sections output deterministic."

The xxHash64 function has been made unsigned-char-independent, so
we can reland this change now.

Original commit message:
> The icf-safe.s test currently fails on 32-bit platforms because it uses
> the --print-icf-sections flag and depends on the output appearing in
> a specific order. However, this flag causes the output to depend on
> the order of the sections in the Sections array, which depends on the
> hash values returned from hash_combine, which happen to be different
> for that test between 32-bit and 64-bit platforms.
>
> This change makes the output deterministic by using xxHash64 instead of
> hash_combine.

Differential Revision: https://reviews.llvm.org/D49877

llvm-svn: 338153

lld/ELF/ICF.cpp

@@ -80,9 +80,10 @@
 #include "SyntheticSections.h"
 #include "Writer.h"
 #include "lld/Common/Threads.h"
-#include "llvm/ADT/Hashing.h"
+#include "llvm/ADT/StringExtras.h"
 #include "llvm/BinaryFormat/ELF.h"
 #include "llvm/Object/ELF.h"
+#include "llvm/Support/xxhash.h"
 #include <algorithm>
 #include <atomic>
 
@@ -155,12 +156,6 @@ private:
 };
 }
 
-// Returns a hash value for S. Note that the information about
-// relocation targets is not included in the hash value.
-template <class ELFT> static uint32_t getHash(InputSection *S) {
-  return hash_combine(S->Flags, S->getSize(), S->NumRelocations, S->Data);
-}
-
 // Returns true if section S is subject of ICF.
 static bool isEligible(InputSection *S) {
   if (!S->Live || S->KeepUnique || !(S->Flags & SHF_ALLOC))
@@ -441,7 +436,7 @@ template <class ELFT> void ICF<ELFT>::run() {
   // Initially, we use hash values to partition sections.
   parallelForEach(Sections, [&](InputSection *S) {
     // Set MSB to 1 to avoid collisions with non-hash IDs.
-    S->Class[0] = getHash<ELFT>(S) | (1U << 31);
+    S->Class[0] = xxHash64(toStringRef(S->Data)) | (1U << 31);
   });
 
   // From now on, sections in Sections vector are ordered so that sections

lld/test/ELF/icf-safe.s

@@ -10,10 +10,6 @@
 # RUN: ld.lld %t1.o %t2.o -o %t2 --icf=all --print-icf-sections --export-dynamic | FileCheck --check-prefix=ALL-EXPORT %s
 # RUN: ld.lld %t1copy.o -o %t4 --icf=safe 2>&1 | FileCheck --check-prefix=OBJCOPY %s
 
-# CHECK-NOT: selected section {{.*}}:(.rodata.l1)
-# CHECK: selected section {{.*}}:(.rodata.l3)
-# CHECK:   removing identical section {{.*}}:(.rodata.l4)
-
 # CHECK-NOT: selected section {{.*}}:(.text.f1)
 # CHECK: selected section {{.*}}:(.text.f3)
 # CHECK:   removing identical section {{.*}}:(.text.f4)
@@ -22,6 +18,10 @@
 # CHECK: selected section {{.*}}:(.rodata.h3)
 # CHECK:   removing identical section {{.*}}:(.rodata.h4)
 
+# CHECK-NOT: selected section {{.*}}:(.rodata.l1)
+# CHECK: selected section {{.*}}:(.rodata.l3)
+# CHECK:   removing identical section {{.*}}:(.rodata.l4)
+
 # CHECK-NOT: selected section {{.*}}:(.rodata.g1)
 # CHECK: selected section {{.*}}:(.rodata.g3)
 # CHECK:   removing identical section {{.*}}:(.rodata.g4)
@@ -30,26 +30,26 @@
 
 # With --icf=all address-significance implies keep-unique only for rodata, not
 # text.
-# ALL-NOT: selected section {{.*}}:(.rodata.l1)
-# ALL: selected section {{.*}}:(.rodata.l3)
-# ALL:   removing identical section {{.*}}:(.rodata.l4)
-
 # ALL: selected section {{.*}}:(.text.f3)
 # ALL:   removing identical section {{.*}}:(.text.f4)
 
-# ALL: selected section {{.*}}:(.text.f1)
-# ALL:   removing identical section {{.*}}:(.text.f2)
-# ALL:   removing identical section {{.*}}:(.text.non_addrsig1)
-# ALL:   removing identical section {{.*}}:(.text.non_addrsig2)
-
 # ALL-NOT: selected section {{.*}}:(.rodata.h1)
 # ALL: selected section {{.*}}:(.rodata.h3)
 # ALL:   removing identical section {{.*}}:(.rodata.h4)
 
+# ALL-NOT: selected section {{.*}}:(.rodata.l1)
+# ALL: selected section {{.*}}:(.rodata.l3)
+# ALL:   removing identical section {{.*}}:(.rodata.l4)
+
 # ALL-NOT: selected section {{.*}}:(.rodata.g1)
 # ALL: selected section {{.*}}:(.rodata.g3)
 # ALL:   removing identical section {{.*}}:(.rodata.g4)
 
+# ALL: selected section {{.*}}:(.text.f1)
+# ALL:   removing identical section {{.*}}:(.text.f2)
+# ALL:   removing identical section {{.*}}:(.text.non_addrsig1)
+# ALL:   removing identical section {{.*}}:(.text.non_addrsig2)
+
 # llvm-mc normally emits an empty .text section into every object file. Since
 # nothing actually refers to it via a relocation, it doesn't have any associated
 # symbols (thus nor can anything refer to it via a relocation, making it safe to
@@ -58,36 +58,36 @@
 # STB_LOCAL or STV_HIDDEN symbols. The dynsym entries should have prevented
 # anything else from being merged.
 # EXPORT-NOT: selected section
-# EXPORT: selected section {{.*}}:(.rodata.l3)
-# EXPORT:   removing identical section {{.*}}:(.rodata.l4)
-# EXPORT-NOT: selected section
 # EXPORT: selected section {{.*}}:(.rodata.h3)
 # EXPORT:   removing identical section {{.*}}:(.rodata.h4)
 # EXPORT-NOT: selected section
 # EXPORT: selected section {{.*}}:(.text)
 # EXPORT:   removing identical section {{.*}}:(.text)
 # EXPORT-NOT: selected section
+# EXPORT: selected section {{.*}}:(.rodata.l3)
+# EXPORT:   removing identical section {{.*}}:(.rodata.l4)
+# EXPORT-NOT: selected section
 
 # If --icf=all is specified when exporting we can also merge the exported text
 # sections, but not the exported rodata.
 # ALL-EXPORT-NOT: selected section
-# ALL-EXPORT: selected section {{.*}}:(.rodata.l3)
-# ALL-EXPORT:   removing identical section {{.*}}:(.rodata.l4)
-# ALL-EXPORT-NOT: selected section
 # ALL-EXPORT: selected section {{.*}}:(.text.f3)
 # ALL-EXPORT:   removing identical section {{.*}}:(.text.f4)
 # ALL-EXPORT-NOT: selected section
-# ALL-EXPORT: selected section {{.*}}:(.text.f1)
-# ALL-EXPORT:   removing identical section {{.*}}:(.text.f2)
-# ALL-EXPORT:   removing identical section {{.*}}:(.text.non_addrsig1)
-# ALL-EXPORT:   removing identical section {{.*}}:(.text.non_addrsig2)
-# ALL-EXPORT-NOT: selected section
 # ALL-EXPORT: selected section {{.*}}:(.rodata.h3)
 # ALL-EXPORT:   removing identical section {{.*}}:(.rodata.h4)
 # ALL-EXPORT-NOT: selected section
 # ALL-EXPORT: selected section {{.*}}:(.text)
 # ALL-EXPORT:   removing identical section {{.*}}:(.text)
 # ALL-EXPORT-NOT: selected section
+# ALL-EXPORT: selected section {{.*}}:(.rodata.l3)
+# ALL-EXPORT:   removing identical section {{.*}}:(.rodata.l4)
+# ALL-EXPORT-NOT: selected section
+# ALL-EXPORT: selected section {{.*}}:(.text.f1)
+# ALL-EXPORT:   removing identical section {{.*}}:(.text.f2)
+# ALL-EXPORT:   removing identical section {{.*}}:(.text.non_addrsig1)
+# ALL-EXPORT:   removing identical section {{.*}}:(.text.non_addrsig2)
+# ALL-EXPORT-NOT: selected section
 
 # OBJCOPY: --icf=safe is incompatible with object files created using objcopy or ld -r