[llvm] 47e2faf - [X86] Do not allow FixupSetCC to relax constraints

[Harald van Dijk via llvm-commits]

Author: Harald van Dijk
Date: 2020-11-28T17:46:56Z
New Revision: 47e2fafbf3d933532f46ef6e8515e7005df52758

URL: https://github.com/llvm/llvm-project/commit/47e2fafbf3d933532f46ef6e8515e7005df52758
DIFF: https://github.com/llvm/llvm-project/commit/47e2fafbf3d933532f46ef6e8515e7005df52758.diff

LOG: [X86] Do not allow FixupSetCC to relax constraints

The build bots caught two additional pre-existing problems exposed by the test change part of my change https://reviews.llvm.org/D91339, when expensive checks are enabled. https://reviews.llvm.org/D91924 fixes one of them, this fixes the other.

FixupSetCC will change code in the form of

  %setcc = SETCCr ...
  %ext1 = MOVZX32rr8 %setcc

to

  %zero = MOV32r0
  %setcc = SETCCr ...
  %ext2 = INSERT_SUBREG %zero, %setcc, %subreg.sub_8bit

and replace uses of %ext1 with %ext2.

The register class for %ext2 did not take into account any constraints on %ext1, which may have been required by its uses. This change ensures that the original constraints are honoured, by instead of creating a new %ext2 register, reusing %ext1 and further constraining it as needed. This requires a slight reorganisation to account for the fact that it is possible for the constraining to fail, in which case no changes should be made.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D91933

Added: 
    

Modified: 
    llvm/lib/Target/X86/X86FixupSetCC.cpp

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Target/X86/X86FixupSetCC.cpp b/llvm/lib/Target/X86/X86FixupSetCC.cpp
index 09668d7c5468..269f8ce6bd7a 100644
--- a/llvm/lib/Target/X86/X86FixupSetCC.cpp
+++ b/llvm/lib/Target/X86/X86FixupSetCC.cpp
@@ -97,28 +97,31 @@ bool X86FixupSetCCPass::runOnMachineFunction(MachineFunction &MF) {
       if (FlagsDefMI->readsRegister(X86::EFLAGS))
         continue;
 
-      ++NumSubstZexts;
-      Changed = true;
-
       // On 32-bit, we need to be careful to force an ABCD register.
       const TargetRegisterClass *RC = MF.getSubtarget<X86Subtarget>().is64Bit()
                                           ? &X86::GR32RegClass
                                           : &X86::GR32_ABCDRegClass;
-      Register ZeroReg = MRI->createVirtualRegister(RC);
-      Register InsertReg = MRI->createVirtualRegister(RC);
+      if (!MRI->constrainRegClass(ZExt->getOperand(0).getReg(), RC)) {
+        // If we cannot constrain the register, we would need an additional copy
+        // and are better off keeping the MOVZX32rr8 we have now.
+        continue;
+      }
+
+      ++NumSubstZexts;
+      Changed = true;
 
       // Initialize a register with 0. This must go before the eflags def
+      Register ZeroReg = MRI->createVirtualRegister(RC);
       BuildMI(MBB, FlagsDefMI, MI.getDebugLoc(), TII->get(X86::MOV32r0),
               ZeroReg);
 
       // X86 setcc only takes an output GR8, so fake a GR32 input by inserting
       // the setcc result into the low byte of the zeroed register.
       BuildMI(*ZExt->getParent(), ZExt, ZExt->getDebugLoc(),
-              TII->get(X86::INSERT_SUBREG), InsertReg)
+              TII->get(X86::INSERT_SUBREG), ZExt->getOperand(0).getReg())
           .addReg(ZeroReg)
           .addReg(MI.getOperand(0).getReg())
           .addImm(X86::sub_8bit);
-      MRI->replaceRegWith(ZExt->getOperand(0).getReg(), InsertReg);
       ToErase.push_back(ZExt);
     }
   }