[PATCH] D97208: [X86] Always use rip-relative addressing on 64-bit when rematerializing all zeros/ones registers using a folded load.

[Craig Topper via Phabricator via llvm-commits]

craig.topper created this revision.
craig.topper added reviewers: RKSimon, MaskRay, rnk, spatel, pengfei.
Herald added a subscriber: hiraditya.
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Previously we only used RIP relative when PIC was enabled. But
we know we're in small/kernel code model here so we should
be able to always use RIP-relative which will give a smaller
encoding.

None of our lit tests are affected by this for some reason. Maybe the
update_test_checks.py is too aggressive with regular expressions?
I'll work on a proper test before committing, but I wanted to make
sure I wasn't missing anything that would prevent us from making this
change.

Here's a godbolt link that demonstrates the current codegen https://godbolt.org/z/j3158o
Note in the non-PIC version the load from .LCPI0_0 doesn't use
RIP-relative addressing, but if you change the constant in the
source from 0.0 to 1.0 it will become RIP-relative.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D97208

Files:
  llvm/lib/Target/X86/X86InstrInfo.cpp


Index: llvm/lib/Target/X86/X86InstrInfo.cpp
===================================================================
--- llvm/lib/Target/X86/X86InstrInfo.cpp
+++ llvm/lib/Target/X86/X86InstrInfo.cpp
@@ -6084,15 +6084,16 @@
 
     // x86-32 PIC requires a PIC base register for constant pools.
     unsigned PICBase = 0;
-    if (MF.getTarget().isPositionIndependent()) {
-      if (Subtarget.is64Bit())
-        PICBase = X86::RIP;
-      else
-        // FIXME: PICBase = getGlobalBaseReg(&MF);
-        // This doesn't work for several reasons.
-        // 1. GlobalBaseReg may have been spilled.
-        // 2. It may not be live at MI.
-        return nullptr;
+    // Since we're using Small or Kernel code model, we can always use
+    // RIP-relative addressing for a smaller encoding.
+    if (Subtarget.is64Bit()) {
+      PICBase = X86::RIP;
+    } else if (MF.getTarget().isPositionIndependent()) {
+      // FIXME: PICBase = getGlobalBaseReg(&MF);
+      // This doesn't work for several reasons.
+      // 1. GlobalBaseReg may have been spilled.
+      // 2. It may not be live at MI.
+      return nullptr;
     }
 
     // Create a constant-pool entry.


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