Fix System.String over-allocation (#17876)

BaseSize for System.String was not set correctly. It caused unnecessary extra 8 bytes to be allocated at the end of strings that had `Length % 4 < 2` on 64-bit platforms.

This change makes affected strings proportionally cheaper. For example, `new string('a', 1)` in a long-running loop is 7% faster.

1 files changed, 4 insertions, 5 deletions

diff --git a/src/vm/i386/jitinterfacex86.cpp b/src/vm/i386/jitinterfacex86.cpp
index 642c417598..4eccd35a76 100644
--- a/src/vm/i386/jitinterfacex86.cpp
+++ b/src/vm/i386/jitinterfacex86.cpp
@@ -1063,14 +1063,13 @@ void *JIT_TrialAlloc::GenAllocString(Flags flags)
     // jae noLock - seems tempting to jump to noAlloc, but we haven't taken the lock yet
     sl.X86EmitCondJump(noLock, X86CondCode::kJAE);
 
-    // mov edx, [ecx]MethodTable.m_BaseSize
-    sl.X86EmitIndexRegLoad(kEDX, kECX, offsetof(MethodTable,m_BaseSize));
-
     // Calculate the final size to allocate.
     // We need to calculate baseSize + cnt*2, then round that up by adding 3 and anding ~3.
 
-    // lea eax, [edx+eax*2+5]
-    sl.X86EmitOp(0x8d, kEAX, kEDX, (DATA_ALIGNMENT-1), kEAX, 2);
+    // lea eax, [basesize+(alignment-1)+eax*2]
+    sl.Emit16(0x048d);
+    sl.Emit8(0x45);
+    sl.Emit32(StringObject::GetBaseSize() + (DATA_ALIGNMENT-1));
 
     // and eax, ~3
     sl.Emit16(0xe083);