Weight variables by their number of uses for register allocation.

tests_lit/assembler/x86/jump_encodings.ll

 ; Tests various aspects of x86 branch encodings (near vs far,
 ; forward vs backward, using CFG labels, or local labels).
 
 ; Use -ffunction-sections so that the offsets reset for each function.
 ; RUN: %p2i --filetype=obj --disassemble -i %s --args -O2 \
 ; RUN:   -ffunction-sections | FileCheck %s
 
 ; Use atomic ops as filler, which shouldn't get optimized out.
 declare void @llvm.nacl.atomic.store.i32(i32, i32*, i32)
 declare i32 @llvm.nacl.atomic.load.i32(i32*, i32)
@@ skipping 151 matching lines @@
   br label %next
 next:
   %ptr = inttoptr i32 %iptr to i32*
   call void @llvm.nacl.atomic.store.i32(i32 %val, i32* %ptr, i32 6)
   br label %next2
 next2:
   %cmp = icmp ult i64 %val64, 0
   br i1 %cmp, label %next, label %next2
 }
 ; CHECK-LABEL: test_local_forward_then_back
-; CHECK:      14: {{.*}} mov DWORD PTR
-; CHECK-NEXT: 16: {{.*}} mfence
-; CHECK-NEXT: 19: {{.*}} mov DWORD PTR {{.*}},0x1
-; CHECK-NEXT: 20: {{.*}} cmp
-; CHECK-NEXT: 23: {{.*}} jb 33
-; CHECK:      37: {{.*}} jne 14
-; CHECK:      39: {{.*}} jmp 19
+; CHECK:      {{.*}} mov DWORD PTR
+; CHECK-NEXT: {{.*}} mfence
+; CHECK-NEXT: {{.*}} mov {{.*}},0x1

[jvoung (off chromium), 2015/08/28 20:31:45]

Hmm yes, this test is pretty brittle and annoying to update. However, removing
the instruction addresses and jmp targets mean it's not testing anything new
(the "make sure that the instruction size accounting for the forward branches
are correct").

On possible middle-ground is to only check the jmp to %next2 and the address of
%next2.

That is, keep the address of mov {{.*}},0x1 (the first instruction for %next2),
and keep the target of the last jmp (which goes to %next2). For me, the address
is 0x16.

[ascull, 2015/08/28 21:22:36]

Done.

+; CHECK-NEXT: {{.*}} cmp
+; CHECK-NEXT: {{.*}} jb
+; CHECK:      {{.*}} jne
+; CHECK:      {{.*}} jmp
 
 
 ; Test that backward local branches also work and are small.
 ; Some of the atomic instructions use a cmpxchg loop.
 define void @test_local_backward(i64 %val64, i32 %iptr, i32 %val) {
 entry:
   br label %next
 next:
   %ptr = inttoptr i32 %iptr to i32*
   %a = call i32 @llvm.nacl.atomic.rmw.i32(i32 5, i32* %ptr, i32 %val, i32 6)
   br label %next2
 next2:
   %success = icmp eq i32 1, %a
   br i1 %success, label %next, label %next2
 }
 ; CHECK-LABEL: test_local_backward
 ; CHECK:       9: {{.*}} mov {{.*}},DWORD
 ; CHECK:       b: {{.*}} mov
 ; CHECK-NEXT:  d: {{.*}} xor
 ; CHECK-NEXT:  f: {{.*}} lock cmpxchg
 ; CHECK-NEXT: 13: 75 f6 jne b
 ; CHECK:      1c: 74 eb je 9