[x86] use more shift or LEA for select-of-constants

We can convert any select-of-constants to math ops:
http://rise4fun.com/Alive/d7d

For this patch, I'm enhancing an existing x86 transform that uses fake multiplies 
(they always become shl/lea) to avoid cmov or branching. The current code misses 
cases where we have a negative constant and a positive constant, so this is just 
trying to plug that hole.

The DAGCombiner diff prevents us from hitting a terrible inefficiency: we can start 
with a select in IR, create a select DAG node, convert it into a sext, convert it 
back into a select, and then lower it to sext machine code.

Some notes about the test diffs:

1. 2010-08-04-MaskedSignedCompare.ll - We were creating control flow that didn't exist in the IR.
2. memcmp.ll - Choose -1 or 1 is the case that got me looking at this again. I 
   think we could avoid the push/pop in some cases if we used 'movzbl %al' instead of an xor on 
   a different reg? That's a post-DAG problem though.
3. mul-constant-result.ll - The trade-off between sbb+not vs. setne+neg could be addressed if 
   that's a regression, but I think those would always be nearly equivalent.
4. pr22338.ll and sext-i1.ll - These tests have undef operands, so I don't think we actually care about these diffs.
5. sbb.ll - This shows a win for what I think is a common case: choose -1 or 0.
6. select.ll - There's another borderline case here: cmp+sbb+or vs. test+set+lea? Also, sbb+not vs. setae+neg shows up again.
7. select_const.ll - These are motivating cases for the enhancement; replace cmov with cheaper ops.

Assembly differences between movzbl and xor to avoid a partial reg stall are caused later by the X86 Fixup SetCC pass.

Differential Revision: https://reviews.llvm.org/D35340

llvm-svn: 310208

1 files changed, 8 insertions, 10 deletions

diff --git a/llvm/test/CodeGen/X86/2010-08-04-MaskedSignedCompare.ll b/llvm/test/CodeGen/X86/2010-08-04-MaskedSignedCompare.ll
index 66d3f3108ec..cffefc2bee6 100644
--- a/llvm/test/CodeGen/X86/2010-08-04-MaskedSignedCompare.ll
+++ b/llvm/test/CodeGen/X86/2010-08-04-MaskedSignedCompare.ll
@@ -9,21 +9,19 @@
 define i32 @main() nounwind {
 ; CHECK-LABEL: main:
 ; CHECK:       # BB#0: # %entry
-; CHECK-NEXT:    cmpq $0, {{.*}}(%rip)
-; CHECK-NEXT:    movb $-106, %al
-; CHECK-NEXT:    jne .LBB0_2
-; CHECK-NEXT:  # BB#1: # %entry
 ; CHECK-NEXT:    xorl %eax, %eax
-; CHECK-NEXT:  .LBB0_2: # %entry
+; CHECK-NEXT:    cmpq {{.*}}(%rip), %rax
+; CHECK-NEXT:    sbbl %eax, %eax
+; CHECK-NEXT:    andl $150, %eax
 ; CHECK-NEXT:    testb %al, %al
-; CHECK-NEXT:    jle .LBB0_3
-; CHECK-NEXT:  # BB#4: # %if.then
+; CHECK-NEXT:    jle .LBB0_1
+; CHECK-NEXT:  # BB#2: # %if.then
 ; CHECK-NEXT:    movl $1, {{.*}}(%rip)
 ; CHECK-NEXT:    movl $1, %esi
-; CHECK-NEXT:    jmp .LBB0_5
-; CHECK-NEXT:  .LBB0_3: # %entry.if.end_crit_edge
+; CHECK-NEXT:    jmp .LBB0_3
+; CHECK-NEXT:  .LBB0_1: # %entry.if.end_crit_edge
 ; CHECK-NEXT:    movl {{.*}}(%rip), %esi
-; CHECK-NEXT:  .LBB0_5: # %if.end
+; CHECK-NEXT:  .LBB0_3: # %if.end
 ; CHECK-NEXT:    pushq %rax
 ; CHECK-NEXT:    movl $.L.str, %edi
 ; CHECK-NEXT:    xorl %eax, %eax