Changes in directory llvm/lib/Target/X86:
README.txt updated: 1.92 -> 1.93
---
Log message:
Add a note.
---
Diffs of the changes: (+58 -0)
README.txt | 58 ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
1 files changed, 58 insertions(+)
Index: llvm/lib/Target/X86/README.txt
diff -u llvm/lib/Target/X86/README.txt:1.92 llvm/lib/Target/X86/README.txt:1.93
--- llvm/lib/Target/X86/README.txt:1.92 Mon Apr 17 22:45:01 2006
+++ llvm/lib/Target/X86/README.txt Wed Apr 19 00:53:27 2006
@@ -996,3 +996,61 @@
movaps %xmm3, %xmm2
movaps %xmm4, %xmm3
jne LBB_main_4 # cond_true44
+
+//===---------------------------------------------------------------------===//
+
+Use the 0's in the top part of movss from memory (and from other instructions
+that generate them) to build vectors more efficiently. Consider:
+
+vector float test(float a) {
+ return (vector float){ 0.0, a, 0.0, 0.0};
+}
+
+We currently generate this as:
+
+_test:
+ sub %ESP, 28
+ movss %XMM0, DWORD PTR [%ESP + 32]
+ movss DWORD PTR [%ESP + 4], %XMM0
+ mov DWORD PTR [%ESP + 12], 0
+ mov DWORD PTR [%ESP + 8], 0
+ mov DWORD PTR [%ESP], 0
+ movaps %XMM0, XMMWORD PTR [%ESP]
+ add %ESP, 28
+ ret
+
+Something like this should be sufficient:
+
+_test:
+ movss %XMM0, DWORD PTR [%ESP + 4]
+ shufps %XMM0, %XMM0, 81
+ ret
+
+... which takes advantage of the zero elements provided by movss.
+Even xoring a register and shufps'ing IT would be better than the
+above code.
+
+Likewise, for this:
+
+vector float test(float a, float b) {
+ return (vector float){ b, a, 0.0, 0.0};
+}
+
+_test:
+ pxor %XMM0, %XMM0
+ movss %XMM1, %XMM0
+ movss %XMM2, DWORD PTR [%ESP + 4]
+ unpcklps %XMM2, %XMM1
+ movss %XMM0, DWORD PTR [%ESP + 8]
+ unpcklps %XMM0, %XMM1
+ unpcklps %XMM0, %XMM2
+ ret
+
+... where we do use pxor, it would be better to use the zero'd
+elements that movss provides to turn this into 2 shufps's instead
+of 3 unpcklps's.
+
+Another example: {0.0, 0.0, a, b }
+
+//===---------------------------------------------------------------------===//
+
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