[llvm] [ARM][Thumb1] Improve EstimateFunctionSizeInBytes accuracy (PR #203318)

[Peter Smith via llvm-commits]

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@@ -2346,16 +2346,146 @@ bool ARMFrameLowering::restoreCalleeSavedRegisters(
 
 // FIXME: Make generic?
 static unsigned EstimateFunctionSizeInBytes(const MachineFunction &MF,
-                                            const ARMBaseInstrInfo &TII) {
+                                            const ARMBaseInstrInfo &TII,
+                                            const ARMSubtarget &STI,
+                                            bool BigFrameOffsets) {
   unsigned FnSize = 0;
+
+  if (MF.shouldSplitStack()) {
+    // Split stack prologue saves r4,r5; makes a copy of sp and loads
+    // a literal; compares the two, and if sp < literal, pushes
+    // further registers and calls __morestack.
+    FnSize += 0x24;
+  }
+
+  // Size of a particularly large Thumb1 stack setup prologue:
+  // update sp for variadic functions (2 bytes)
+  // + push registers (maybe high ones by copying them down, up to 14 bytes)
+  // + frame pointer (might use r11, requiring pushing it first, 6 bytes)
+  // + stack update (up to 6 bytes)
+  // + stack realignment (8)
+  // + make base pointer (2).
+  FnSize += 0x38;
+
+  // Size of a large epilogue:
+  // restore sp from frame pointer (6 bytes if it's in r11)
+  // + pop registers (up to 14 bytes, as above)
+  // + pop r11 if it was saved to make frame pointer (4 bytes)
+  // + pop return address into a low reg (2 bytes)
+  // + update sp to undo variadic function setup (2 bytes)
+  // + BX to where you popped the return address (2 bytes)
+  FnSize += 0x1e;
+
   for (auto &MBB : MF) {
-    for (auto &MI : MBB)
-      FnSize += TII.getInstSizeInBytes(MI);
+    bool seenBranch = false, seenConstantLoad = false;
+    for (auto &MI : MBB) {
+      unsigned InstSize;
+      switch (MI.getOpcode()) {
+      case ARM::tADDframe:
+        if (BigFrameOffsets) {
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smithp35 wrote:

Coding style nit. I think the LLVM coding standard prefers not to have braces around single statements. 

I don't personally have a strong opinion on this, but could be worth a check.

https://github.com/llvm/llvm-project/pull/203318