Changes in directory llvm/lib/Target/X86:
README.txt updated: 1.157 -> 1.158
---
Log message:
Notes about codegen issues.
---
Diffs of the changes: (+47 -0)
README.txt | 47 +++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 47 insertions(+)
Index: llvm/lib/Target/X86/README.txt
diff -u llvm/lib/Target/X86/README.txt:1.157
llvm/lib/Target/X86/README.txt:1.158
--- llvm/lib/Target/X86/README.txt:1.157 Thu Mar 1 23:04:52 2007
+++ llvm/lib/Target/X86/README.txt Wed Mar 14 16:03:53 2007
@@ -339,6 +339,53 @@
//===---------------------------------------------------------------------===//
+We are generating far worse code than gcc:
+
+volatile short X, Y;
+
+void foo(int N) {
+ int i;
+ for (i = 0; i < N; i++) { X = i; Y = i*4; }
+}
+
+LBB1_1: #bb.preheader
+ xorl %ecx, %ecx
+ xorw %dx, %dx
+LBB1_2: #bb
+ movl L_X$non_lazy_ptr, %esi
+ movw %dx, (%esi)
+ movw %dx, %si
+ shlw $2, %si
+ movl L_Y$non_lazy_ptr, %edi
+ movw %si, (%edi)
+ incl %ecx
+ incw %dx
+ cmpl %eax, %ecx
+ jne LBB1_2 #bb
+
+vs.
+
+ xorl %edx, %edx
+ movl L_X$non_lazy_ptr-"L00000000001$pb"(%ebx), %esi
+ movl L_Y$non_lazy_ptr-"L00000000001$pb"(%ebx), %ecx
+L4:
+ movw %dx, (%esi)
+ leal 0(,%edx,4), %eax
+ movw %ax, (%ecx)
+ addl $1, %edx
+ cmpl %edx, %edi
+ jne L4
+
+There are 3 issues:
+
+1. Lack of post regalloc LICM.
+2. Poor sub-regclass support. That leads to inability to promote the 16-bit
+ arithmetic op to 32-bit and making use of leal.
+3. LSR unable to reused IV for a different type (i16 vs. i32) even though
+ the cast would be free.
+
+//===---------------------------------------------------------------------===//
+
Teach the coalescer to coalesce vregs of different register classes. e.g. FR32
/
FR64 to VR128.
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