Index: clang/lib/CodeGen/CGCall.cpp
===================================================================
--- clang/lib/CodeGen/CGCall.cpp
+++ clang/lib/CodeGen/CGCall.cpp
@@ -2334,6 +2334,16 @@
       RetAttrs.addAttribute(llvm::Attribute::NoUndef);
   }
 
+  auto AddArm64ECArgSize = [this](QualType Ty, llvm::AttrBuilder &Attrs) {
+    if (Ty->isRecordType()) {
+      CharUnits SizeInBytes = getContext().getTypeSizeInChars(Ty.getTypePtr());
+      Attrs.addArm64ECArgSizeAttr(SizeInBytes.getQuantity());
+    }
+  };
+
+  if (getTriple().isWindowsArm64EC())
+    AddArm64ECArgSize(RetTy, RetAttrs);
+
   switch (RetAI.getKind()) {
   case ABIArgInfo::Extend:
     if (RetAI.isSignExt())
@@ -2467,6 +2477,9 @@
       Attrs.addAttribute(llvm::Attribute::NoUndef);
     }
 
+    if (getTriple().isWindowsArm64EC())
+      AddArm64ECArgSize(ParamType, Attrs);
+
     // 'restrict' -> 'noalias' is done in EmitFunctionProlog when we
     // have the corresponding parameter variable.  It doesn't make
     // sense to do it here because parameters are so messed up.
Index: clang/test/CodeGenCXX/arm64ec-mangle-align.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/arm64ec-mangle-align.cpp
@@ -0,0 +1,180 @@
+// RUN: %clang_cc1 %s -triple=arm64ec-pc-windows-msvc -emit-llvm -fms-compatibility -o - | FileCheck %s
+
+#pragma pack(push, 1)
+
+struct __declspec(align(1)) s1 {
+  char a[1];
+};
+s1 (*pfnstruct1)(s1 x);
+s1 callstruct1(s1 x) { return pfnstruct1(x); };
+struct __declspec(align(8)) s2 {
+  char a[2];
+};
+s2 (*pfnstruct2)(s2 x);
+s2 callstruct2(s2 x) { return pfnstruct2(x); };
+struct __declspec(align(64)) s3 {
+  char a[3];
+};
+s3 (*pfnstruct3)(s3 x);
+s3 callstruct3(s3 x) { return pfnstruct3(x); };
+struct __declspec(align(32)) s4 {
+  char a[4];
+};
+s4 (*pfnstruct4)(s4 x);
+s4 callstruct4(s4 x) { return pfnstruct4(x); };
+struct __declspec(align(2)) s5 {
+  char a[5];
+};
+s5 (*pfnstruct5)(s5 x);
+s5 callstruct5(s5 x) { return pfnstruct5(x); };
+struct __declspec(align(4)) s6 {
+  char a[6];
+};
+s6 (*pfnstruct6)(s6 x);
+s6 callstruct6(s6 x) { return pfnstruct6(x); };
+struct __declspec(align(8)) s7 {
+  char a[7];
+};
+s7 (*pfnstruct7)(s7 x);
+s7 callstruct7(s7 x) { return pfnstruct7(x); };
+struct __declspec(align(16)) s8 {
+  char a[8];
+};
+s8 (*pfnstruct8)(s8 x);
+s8 callstruct8(s8 x) { return pfnstruct8(x); };
+struct __declspec(align(4)) s9 {
+  char a[9];
+};
+s9 (*pfnstruct9)(s9 x);
+s9 callstruct9(s9 x) { return pfnstruct9(x); };
+struct __declspec(align(2)) s10 {
+  char a[10];
+};
+s10 (*pfnstruct10)(s10 x);
+s10 callstruct10(s10 x) { return pfnstruct10(x); };
+struct __declspec(align(1)) s11 {
+  char a[11];
+};
+s11 (*pfnstruct11)(s11 x);
+s11 callstruct11(s11 x) { return pfnstruct11(x); };
+struct __declspec(align(64)) s12 {
+  char a[12];
+};
+s12 (*pfnstruct12)(s12 x);
+s12 callstruct12(s12 x) { return pfnstruct12(x); };
+struct __declspec(align(8)) s13 {
+  char a[13];
+};
+s13 (*pfnstruct13)(s13 x);
+s13 callstruct13(s13 x) { return pfnstruct13(x); };
+struct __declspec(align(4)) s14 {
+  char a[14];
+};
+s14 (*pfnstruct14)(s14 x);
+s14 callstruct14(s14 x) { return pfnstruct14(x); };
+struct __declspec(align(16)) s15 {
+  char a[15];
+};
+s15 (*pfnstruct15)(s15 x);
+s15 callstruct15(s15 x) { return pfnstruct15(x); };
+struct __declspec(align(32)) s16 {
+  char a[16];
+};
+s16 (*pfnstruct16)(s16 x);
+s16 callstruct16(s16 x) { return pfnstruct16(x); };
+struct __declspec(align(64)) s17 {
+  char a[17];
+};
+s17 (*pfnstruct17)(s17 x);
+s17 callstruct17(s17 x) { return pfnstruct17(x); };
+struct __declspec(align(4)) s33 {
+  char a[33];
+};
+s33 (*pfnstruct33)(s33 x);
+s33 callstruct33(s33 x) { return pfnstruct33(x); };
+struct __declspec(align(2)) s65 {
+  char a[65];
+};
+s65 (*pfnstruct65)(s65 x);
+s65 callstruct65(s65 x) { return pfnstruct65(x); };
+struct __declspec(align(16)) s129 {
+  char a[129];
+};
+s129 (*pfnstruct129)(s129 x);
+s129 callstruct129(s129 x) { return pfnstruct129(x); };
+struct __declspec(align(32)) s257 {
+  char a[257];
+};
+s257 (*pfnstruct257)(s257 x);
+s257 callstruct257(s257 x) { return pfnstruct257(x); };
+struct __declspec(align(8)) f2 {
+  float a[2];
+};
+f2 (*pfnstructf2)(f2 x);
+f2 callstructf2(f2 x) { return pfnstructf2(x); };
+struct __declspec(align(64)) f3 {
+  float a[3];
+};
+f3 (*pfnstructf3)(f3 x);
+f3 callstructf3(f3 x) { return pfnstructf3(x); };
+struct __declspec(align(32)) f4 {
+  float a[4];
+};
+f4 (*pfnstructf4)(f4 x);
+f4 callstructf4(f4 x) { return pfnstructf4(x); };
+struct __declspec(align(32)) f5 {
+  float a[5];
+};
+f5 (*pfnstructf5)(f5 x);
+f5 callstructf5(f5 x) { return pfnstructf5(x); };
+struct __declspec(align(64))  d2 {
+  double a[2];
+};
+d2 (*pfnstructd2)(d2 x);
+d2 callstructd2(d2 x) { return pfnstructd2(x); };
+struct __declspec(align(64)) d3 {
+  double a[3];
+};
+d3 (*pfnstructd3)(d3 x);
+d3 callstructd3(d3 x) { return pfnstructd3(x); };
+struct __declspec(align(16)) d4 {
+  double a[4];
+};
+d4 (*pfnstructd4)(d4 x);
+d4 callstructd4(d4 x) { return pfnstructd4(x); };
+struct __declspec(align(16)) d5 {
+  double a[5];
+};
+d5 (*pfnstructd5)(d5 x);
+d5 callstructd5(d5 x) { return pfnstructd5(x); };
+#pragma pack(pop)
+
+// CHECK: define dso_local arm64ec_argsize(1) i8 @"?callstruct1@@$$hYA?AUs1@@U1@@Z"(i64 arm64ec_argsize(1) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(8) i64 @"?callstruct2@@$$hYA?AUs2@@U1@@Z"(i64 arm64ec_argsize(8) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstruct3@@$$hYA?AUs3@@U1@@Z"(ptr noalias sret(%struct.s3) align 64 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(32) void @"?callstruct4@@$$hYA?AUs4@@U1@@Z"(ptr noalias sret(%struct.s4) align 32 %agg.result, ptr noundef arm64ec_argsize(32) %x)
+// CHECK: define dso_local arm64ec_argsize(6) i48 @"?callstruct5@@$$hYA?AUs5@@U1@@Z"(i64 arm64ec_argsize(6) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(8) i64 @"?callstruct6@@$$hYA?AUs6@@U1@@Z"(i64 arm64ec_argsize(8) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(8) i64 @"?callstruct7@@$$hYA?AUs7@@U1@@Z"(i64 arm64ec_argsize(8) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(16) i128 @"?callstruct8@@$$hYA?AUs8@@U1@@Z"(i128 arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(12) [2 x i64] @"?callstruct9@@$$hYA?AUs9@@U1@@Z"([2 x i64] arm64ec_argsize(12) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(10) [2 x i64] @"?callstruct10@@$$hYA?AUs10@@U1@@Z"([2 x i64] arm64ec_argsize(10) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(11) [2 x i64] @"?callstruct11@@$$hYA?AUs11@@U1@@Z"([2 x i64] arm64ec_argsize(11) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstruct12@@$$hYA?AUs12@@U1@@Z"(ptr noalias sret(%struct.s12) align 64 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(16) [2 x i64] @"?callstruct13@@$$hYA?AUs13@@U1@@Z"([2 x i64] arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(16) [2 x i64] @"?callstruct14@@$$hYA?AUs14@@U1@@Z"([2 x i64] arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(16) i128 @"?callstruct15@@$$hYA?AUs15@@U1@@Z"(i128 arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(32) void @"?callstruct16@@$$hYA?AUs16@@U1@@Z"(ptr noalias sret(%struct.s16) align 32 %agg.result, ptr noundef arm64ec_argsize(32) %x)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstruct17@@$$hYA?AUs17@@U1@@Z"(ptr noalias sret(%struct.s17) align 64 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(36) void @"?callstruct33@@$$hYA?AUs33@@U1@@Z"(ptr noalias sret(%struct.s33) align 4 %agg.result, ptr noundef arm64ec_argsize(36) %x)
+// CHECK: define dso_local arm64ec_argsize(66) void @"?callstruct65@@$$hYA?AUs65@@U1@@Z"(ptr noalias sret(%struct.s65) align 2 %agg.result, ptr noundef arm64ec_argsize(66) %x)
+// CHECK: define dso_local arm64ec_argsize(144) void @"?callstruct129@@$$hYA?AUs129@@U1@@Z"(ptr noalias sret(%struct.s129) align 16 %agg.result, ptr noundef arm64ec_argsize(144) %x)
+// CHECK: define dso_local arm64ec_argsize(288) void @"?callstruct257@@$$hYA?AUs257@@U1@@Z"(ptr noalias sret(%struct.s257) align 32 %agg.result, ptr noundef arm64ec_argsize(288) %x)
+// CHECK: define dso_local arm64ec_argsize(8) %struct.f2 @"?callstructf2@@$$hYA?AUf2@@U1@@Z"([2 x float] arm64ec_argsize(8) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstructf3@@$$hYA?AUf3@@U1@@Z"(ptr noalias sret(%struct.f3) align 64 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(32) void @"?callstructf4@@$$hYA?AUf4@@U1@@Z"(ptr noalias sret(%struct.f4) align 32 %agg.result, ptr noundef arm64ec_argsize(32) %x)
+// CHECK: define dso_local arm64ec_argsize(32) void @"?callstructf5@@$$hYA?AUf5@@U1@@Z"(ptr noalias sret(%struct.f5) align 32 %agg.result, ptr noundef arm64ec_argsize(32) %x)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstructd2@@$$hYA?AUd2@@U1@@Z"(ptr noalias sret(%struct.d2) align 64 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstructd3@@$$hYA?AUd3@@U1@@Z"(ptr noalias sret(%struct.d3) align 64 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(32) %struct.d4 @"?callstructd4@@$$hYA?AUd4@@U1@@Z"([4 x double] arm64ec_argsize(32) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(48) void @"?callstructd5@@$$hYA?AUd5@@U1@@Z"(ptr noalias sret(%struct.d5) align 16 %agg.result, ptr noundef arm64ec_argsize(48) %x)
Index: clang/test/CodeGenCXX/arm64ec-mangle-basic.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/arm64ec-mangle-basic.cpp
@@ -0,0 +1,38 @@
+// RUN: %clang_cc1 %s -triple=arm64ec-pc-windows-msvc -emit-llvm -fms-compatibility -o - | FileCheck %s
+
+#pragma pack(push, 1)
+typedef long long i64;
+typedef long double longdouble;
+typedef void *VOIDP;
+bool (*pfnbool)(bool x);
+bool callbool(bool x) { return pfnbool(x); };
+char (*pfnchar)(char x);
+char callchar(char x) { return pfnchar(x); };
+short (*pfnshort)(short x);
+short callshort(short x) { return pfnshort(x); };
+wchar_t (*pfnwchar_t)(wchar_t x);
+wchar_t callwchar_t(wchar_t x) { return pfnwchar_t(x); };
+int (*pfnint)(int x);
+int callint(int x) { return pfnint(x); };
+i64 (*pfni64)(i64 x);
+i64 calli64(i64 x) { return pfni64(x); };
+float (*pfnfloat)(float x);
+float callfloat(float x) { return pfnfloat(x); };
+double (*pfndouble)(double x);
+double calldouble(double x) { return pfndouble(x); };
+longdouble (*pfnlongdouble)(longdouble x);
+longdouble calllongdouble(longdouble x) { return pfnlongdouble(x); };
+VOIDP (*pfnVOIDP)(VOIDP x);
+VOIDP callVOIDP(VOIDP x) { return pfnVOIDP(x); };
+#pragma pack(pop)
+
+// CHECK: define dso_local noundef i1 @"?callbool@@$$hYA_N_N@Z"(i1 noundef %x)
+// CHECK: define dso_local noundef i8 @"?callchar@@$$hYADD@Z"(i8 noundef %x)
+// CHECK: define dso_local noundef i16 @"?callshort@@$$hYAFF@Z"(i16 noundef %x)
+// CHECK: define dso_local noundef i16 @"?callwchar_t@@$$hYA_W_W@Z"(i16 noundef %x)
+// CHECK: define dso_local noundef i32 @"?callint@@$$hYAHH@Z"(i32 noundef %x)
+// CHECK: define dso_local noundef i64 @"?calli64@@$$hYA_J_J@Z"(i64 noundef %x)
+// CHECK: define dso_local noundef float @"?callfloat@@$$hYAMM@Z"(float noundef %x)
+// CHECK: define dso_local noundef double @"?calldouble@@$$hYANN@Z"(double noundef %x)
+// CHECK: define dso_local noundef double @"?calllongdouble@@$$hYAOO@Z"(double noundef %x)
+// CHECK: define dso_local noundef ptr @"?callVOIDP@@$$hYAPEAXPEAX@Z"(ptr noundef %x)
Index: clang/test/CodeGenCXX/arm64ec-mangle-struct.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/arm64ec-mangle-struct.cpp
@@ -0,0 +1,215 @@
+// RUN: %clang_cc1 %s -triple=arm64ec-pc-windows-msvc -emit-llvm -fms-compatibility -o - | FileCheck %s
+
+#pragma pack(push, 1)
+struct s1 {
+  char a[1];
+};
+s1 (*pfnstruct1)(s1 x);
+s1 callstruct1(s1 x) { return pfnstruct1(x); };
+struct s2 {
+  char a[2];
+};
+s2 (*pfnstruct2)(s2 x);
+s2 callstruct2(s2 x) { return pfnstruct2(x); };
+struct s3 {
+  char a[3];
+};
+s3 (*pfnstruct3)(s3 x);
+s3 callstruct3(s3 x) { return pfnstruct3(x); };
+struct s4 {
+  char a[4];
+};
+s4 (*pfnstruct4)(s4 x);
+s4 callstruct4(s4 x) { return pfnstruct4(x); };
+struct s5 {
+  char a[5];
+};
+s5 (*pfnstruct5)(s5 x);
+s5 callstruct5(s5 x) { return pfnstruct5(x); };
+struct s6 {
+  char a[6];
+};
+s6 (*pfnstruct6)(s6 x);
+s6 callstruct6(s6 x) { return pfnstruct6(x); };
+struct s7 {
+  char a[7];
+};
+s7 (*pfnstruct7)(s7 x);
+s7 callstruct7(s7 x) { return pfnstruct7(x); };
+struct s8 {
+  char a[8];
+};
+s8 (*pfnstruct8)(s8 x);
+s8 callstruct8(s8 x) { return pfnstruct8(x); };
+struct s9 {
+  char a[9];
+};
+s9 (*pfnstruct9)(s9 x);
+s9 callstruct9(s9 x) { return pfnstruct9(x); };
+struct s10 {
+  char a[10];
+};
+s10 (*pfnstruct10)(s10 x);
+s10 callstruct10(s10 x) { return pfnstruct10(x); };
+struct s11 {
+  char a[11];
+};
+s11 (*pfnstruct11)(s11 x);
+s11 callstruct11(s11 x) { return pfnstruct11(x); };
+struct s12 {
+  char a[12];
+};
+s12 (*pfnstruct12)(s12 x);
+s12 callstruct12(s12 x) { return pfnstruct12(x); };
+struct s13 {
+  char a[13];
+};
+s13 (*pfnstruct13)(s13 x);
+s13 callstruct13(s13 x) { return pfnstruct13(x); };
+struct s14 {
+  char a[14];
+};
+s14 (*pfnstruct14)(s14 x);
+s14 callstruct14(s14 x) { return pfnstruct14(x); };
+struct s15 {
+  char a[15];
+};
+s15 (*pfnstruct15)(s15 x);
+s15 callstruct15(s15 x) { return pfnstruct15(x); };
+struct s16 {
+  char a[16];
+};
+s16 (*pfnstruct16)(s16 x);
+s16 callstruct16(s16 x) { return pfnstruct16(x); };
+struct s17 {
+  char a[17];
+};
+s17 (*pfnstruct17)(s17 x);
+s17 callstruct17(s17 x) { return pfnstruct17(x); };
+struct s32 {
+  char a[32];
+};
+s32 (*pfnstruct32)(s32 x);
+s32 callstruct32(s32 x) { return pfnstruct32(x); };
+struct s33 {
+  char a[33];
+};
+s33 (*pfnstruct33)(s33 x);
+s33 callstruct33(s33 x) { return pfnstruct33(x); };
+struct s64 {
+  char a[64];
+};
+s64 (*pfnstruct64)(s64 x);
+s64 callstruct64(s64 x) { return pfnstruct64(x); };
+struct s65 {
+  char a[65];
+};
+s65 (*pfnstruct65)(s65 x);
+s65 callstruct65(s65 x) { return pfnstruct65(x); };
+struct s128 {
+  char a[128];
+};
+s128 (*pfnstruct128)(s128 x);
+s128 callstruct128(s128 x) { return pfnstruct128(x); };
+struct s133 {
+  char a[133];
+};
+s133 (*pfnstruct133)(s133 x);
+s133 callstruct133(s133 x) { return pfnstruct133(x); };
+struct s192 {
+  char a[192];
+};
+s192 (*pfnstruct192)(s192 x);
+s192 callstruct192(s192 x) { return pfnstruct192(x); };
+struct s223 {
+  char a[223];
+};
+s223 (*pfnstruct223)(s223 x);
+s223 callstruct223(s223 x) { return pfnstruct223(x); };
+struct s256 {
+  char a[256];
+};
+s256 (*pfnstruct256)(s256 x);
+s256 callstruct256(s256 x) { return pfnstruct256(x); };
+struct s257 {
+  char a[257];
+};
+s257 (*pfnstruct257)(s257 x);
+s257 callstruct257(s257 x) { return pfnstruct257(x); };
+struct f2 {
+  float a[2];
+};
+f2 (*pfnstructf2)(f2 x);
+f2 callstructf2(f2 x) { return pfnstructf2(x); };
+struct f3 {
+  float a[3];
+};
+f3 (*pfnstructf3)(f3 x);
+f3 callstructf3(f3 x) { return pfnstructf3(x); };
+struct f4 {
+  float a[4];
+};
+f4 (*pfnstructf4)(f4 x);
+f4 callstructf4(f4 x) { return pfnstructf4(x); };
+struct f5 {
+  float a[5];
+};
+f5 (*pfnstructf5)(f5 x);
+f5 callstructf5(f5 x) { return pfnstructf5(x); };
+struct d2 {
+  double a[2];
+};
+d2 (*pfnstructd2)(d2 x);
+d2 callstructd2(d2 x) { return pfnstructd2(x); };
+struct d3 {
+  double a[3];
+};
+d3 (*pfnstructd3)(d3 x);
+d3 callstructd3(d3 x) { return pfnstructd3(x); };
+struct d4 {
+  double a[4];
+};
+d4 (*pfnstructd4)(d4 x);
+d4 callstructd4(d4 x) { return pfnstructd4(x); };
+struct d5 {
+  double a[5];
+};
+d5 (*pfnstructd5)(d5 x);
+d5 callstructd5(d5 x) { return pfnstructd5(x); };
+#pragma pack(pop)
+
+// CHECK: define dso_local arm64ec_argsize(1) i8 @"?callstruct1@@$$hYA?AUs1@@U1@@Z"(i64 arm64ec_argsize(1) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(2) i16 @"?callstruct2@@$$hYA?AUs2@@U1@@Z"(i64 arm64ec_argsize(2) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(3) i24 @"?callstruct3@@$$hYA?AUs3@@U1@@Z"(i64 arm64ec_argsize(3) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(4) i32 @"?callstruct4@@$$hYA?AUs4@@U1@@Z"(i64 arm64ec_argsize(4) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(5) i40 @"?callstruct5@@$$hYA?AUs5@@U1@@Z"(i64 arm64ec_argsize(5) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(6) i48 @"?callstruct6@@$$hYA?AUs6@@U1@@Z"(i64 arm64ec_argsize(6) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(7) i56 @"?callstruct7@@$$hYA?AUs7@@U1@@Z"(i64 arm64ec_argsize(7) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(8) i64 @"?callstruct8@@$$hYA?AUs8@@U1@@Z"(i64 arm64ec_argsize(8) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(9) [2 x i64] @"?callstruct9@@$$hYA?AUs9@@U1@@Z"([2 x i64] arm64ec_argsize(9) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(10) [2 x i64] @"?callstruct10@@$$hYA?AUs10@@U1@@Z"([2 x i64] arm64ec_argsize(10) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(11) [2 x i64] @"?callstruct11@@$$hYA?AUs11@@U1@@Z"([2 x i64] arm64ec_argsize(11) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(12) [2 x i64] @"?callstruct12@@$$hYA?AUs12@@U1@@Z"([2 x i64] arm64ec_argsize(12) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(13) [2 x i64] @"?callstruct13@@$$hYA?AUs13@@U1@@Z"([2 x i64] arm64ec_argsize(13) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(14) [2 x i64] @"?callstruct14@@$$hYA?AUs14@@U1@@Z"([2 x i64] arm64ec_argsize(14) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(15) [2 x i64] @"?callstruct15@@$$hYA?AUs15@@U1@@Z"([2 x i64] arm64ec_argsize(15) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(16) [2 x i64] @"?callstruct16@@$$hYA?AUs16@@U1@@Z"([2 x i64] arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(17) void @"?callstruct17@@$$hYA?AUs17@@U1@@Z"(ptr noalias sret(%struct.s17) align 1 %agg.result, ptr noundef arm64ec_argsize(17) %x)
+// CHECK: define dso_local arm64ec_argsize(32) void @"?callstruct32@@$$hYA?AUs32@@U1@@Z"(ptr noalias sret(%struct.s32) align 1 %agg.result, ptr noundef arm64ec_argsize(32) %x)
+// CHECK: define dso_local arm64ec_argsize(33) void @"?callstruct33@@$$hYA?AUs33@@U1@@Z"(ptr noalias sret(%struct.s33) align 1 %agg.result, ptr noundef arm64ec_argsize(33) %x)
+// CHECK: define dso_local arm64ec_argsize(64) void @"?callstruct64@@$$hYA?AUs64@@U1@@Z"(ptr noalias sret(%struct.s64) align 1 %agg.result, ptr noundef arm64ec_argsize(64) %x)
+// CHECK: define dso_local arm64ec_argsize(65) void @"?callstruct65@@$$hYA?AUs65@@U1@@Z"(ptr noalias sret(%struct.s65) align 1 %agg.result, ptr noundef arm64ec_argsize(65) %x)
+// CHECK: define dso_local arm64ec_argsize(128) void @"?callstruct128@@$$hYA?AUs128@@U1@@Z"(ptr noalias sret(%struct.s128) align 1 %agg.result, ptr noundef arm64ec_argsize(128) %x)
+// CHECK: define dso_local arm64ec_argsize(133) void @"?callstruct133@@$$hYA?AUs133@@U1@@Z"(ptr noalias sret(%struct.s133) align 1 %agg.result, ptr noundef arm64ec_argsize(133) %x)
+// CHECK: define dso_local arm64ec_argsize(192) void @"?callstruct192@@$$hYA?AUs192@@U1@@Z"(ptr noalias sret(%struct.s192) align 1 %agg.result, ptr noundef arm64ec_argsize(192) %x)
+// CHECK: define dso_local arm64ec_argsize(223) void @"?callstruct223@@$$hYA?AUs223@@U1@@Z"(ptr noalias sret(%struct.s223) align 1 %agg.result, ptr noundef arm64ec_argsize(223) %x)
+// CHECK: define dso_local arm64ec_argsize(256) void @"?callstruct256@@$$hYA?AUs256@@U1@@Z"(ptr noalias sret(%struct.s256) align 1 %agg.result, ptr noundef arm64ec_argsize(256) %x)
+// CHECK: define dso_local arm64ec_argsize(257) void @"?callstruct257@@$$hYA?AUs257@@U1@@Z"(ptr noalias sret(%struct.s257) align 1 %agg.result, ptr noundef arm64ec_argsize(257) %x)
+// CHECK: define dso_local arm64ec_argsize(8) %struct.f2 @"?callstructf2@@$$hYA?AUf2@@U1@@Z"([2 x float] arm64ec_argsize(8) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(12) %struct.f3 @"?callstructf3@@$$hYA?AUf3@@U1@@Z"([3 x float] arm64ec_argsize(12) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(16) %struct.f4 @"?callstructf4@@$$hYA?AUf4@@U1@@Z"([4 x float] arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(20) void @"?callstructf5@@$$hYA?AUf5@@U1@@Z"(ptr noalias sret(%struct.f5) align 1 %agg.result, ptr noundef arm64ec_argsize(20) %x)
+// CHECK: define dso_local arm64ec_argsize(16) %struct.d2 @"?callstructd2@@$$hYA?AUd2@@U1@@Z"([2 x double] arm64ec_argsize(16) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(24) %struct.d3 @"?callstructd3@@$$hYA?AUd3@@U1@@Z"([3 x double] arm64ec_argsize(24) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(32) %struct.d4 @"?callstructd4@@$$hYA?AUd4@@U1@@Z"([4 x double] arm64ec_argsize(32) %x.coerce)
+// CHECK: define dso_local arm64ec_argsize(40) void @"?callstructd5@@$$hYA?AUd5@@U1@@Z"(ptr noalias sret(%struct.d5) align 1 %agg.result, ptr noundef arm64ec_argsize(40) %x)
Index: clang/test/CodeGenCXX/arm64ec.cpp
===================================================================
--- clang/test/CodeGenCXX/arm64ec.cpp
+++ clang/test/CodeGenCXX/arm64ec.cpp
@@ -2,7 +2,7 @@
 
 // CHECK: @"?g@@YAXUA@@UB@@@Z" = alias void ([2 x float], [4 x float]), void ([2 x float], [4 x float])* @"?g@@$$hYAXUA@@UB@@@Z"
 // CHECK: define dso_local void @"?g@@$$hYAXUA@@UB@@@Z"
-// CHECK: call void (i64, ...) @"?f@@YAXUA@@ZZ"(i64 %{{.*}}, %struct.B* noundef %{{.*}})
+// CHECK: call void (i64, ...) @"?f@@YAXUA@@ZZ"(i64 arm64ec_argsize(8) %{{.*}}, %struct.B* noundef arm64ec_argsize(16) %{{.*}})
 typedef struct { float x[2]; } A;
 typedef struct { float x[4]; } B;
 void f(A a, ...);
Index: llvm/include/llvm/Bitcode/LLVMBitCodes.h
===================================================================
--- llvm/include/llvm/Bitcode/LLVMBitCodes.h
+++ llvm/include/llvm/Bitcode/LLVMBitCodes.h
@@ -690,6 +690,7 @@
   ATTR_KIND_PRESPLIT_COROUTINE = 83,
   ATTR_KIND_FNRETTHUNK_EXTERN = 84,
   ATTR_KIND_SKIP_PROFILE = 85,
+  ATTR_KIND_ARM64EC_ARG_SIZE = 86,
 };
 
 enum ComdatSelectionKindCodes {
Index: llvm/include/llvm/IR/Attributes.h
===================================================================
--- llvm/include/llvm/IR/Attributes.h
+++ llvm/include/llvm/IR/Attributes.h
@@ -145,6 +145,8 @@
   static Attribute getWithPreallocatedType(LLVMContext &Context, Type *Ty);
   static Attribute getWithInAllocaType(LLVMContext &Context, Type *Ty);
   static Attribute getWithUWTableKind(LLVMContext &Context, UWTableKind Kind);
+  static Attribute getWithArm64ECArgSizeBytes(LLVMContext &Context,
+                                              uint64_t Bytes);
 
   /// For a typed attribute, return the equivalent attribute with the type
   /// changed to \p ReplacementTy.
@@ -244,6 +246,9 @@
   // Returns the allocator function kind.
   AllocFnKind getAllocKind() const;
 
+  /// Returns arm64ec x86 signature arg size in bytes
+  uint64_t getArm64ECArgSizeBytes() const;
+
   /// The Attribute is converted to a string of equivalent mnemonic. This
   /// is, presumably, for writing out the mnemonics for the assembly writer.
   std::string getAsString(bool InAttrGrp = false) const;
@@ -376,6 +381,7 @@
   Optional<unsigned> getVScaleRangeMax() const;
   UWTableKind getUWTableKind() const;
   AllocFnKind getAllocKind() const;
+  uint64_t getArm64ECArgSizeBytes() const;
   std::string getAsString(bool InAttrGrp = false) const;
 
   /// Return true if this attribute set belongs to the LLVMContext.
@@ -875,6 +881,13 @@
 
   AllocFnKind getAllocKind() const;
 
+  /// Get the number of Arm64ECArgSize bytes (or zero if unknown) of the
+  /// return value.
+  uint64_t getRetArm64ECArgSizeBytes() const;
+
+  /// Get the number of Arm64ECArgSize bytes (or zero if unknown) of an arg.
+  uint64_t getParamArm64ECArgSizeBytes(unsigned Index) const;
+
   /// Return the attributes at the index as a string.
   std::string getAsString(unsigned Index, bool InAttrGrp = false) const;
 
@@ -1155,6 +1168,11 @@
   /// Retrieve the maximum value of 'vscale_range' or None when unknown.
   Optional<unsigned> getVScaleRangeMax() const;
 
+  /// Retrieve the arm64ec x86 signature size attribute, if it exists.
+  uint64_t getArm64ECArgSizeBytes() const {
+    return getRawIntAttr(Attribute::Arm64ECArgSize);
+  }
+
   /// Add integer attribute with raw value (packed/encoded if necessary).
   AttrBuilder &addRawIntAttr(Attribute::AttrKind Kind, uint64_t Value);
 
@@ -1224,6 +1242,10 @@
   /// Attribute.getIntValue().
   AttrBuilder &addVScaleRangeAttrFromRawRepr(uint64_t RawVScaleRangeRepr);
 
+  /// This turns the number of arm64ec args with x86 signature bytes into the
+  /// form used internally in Attribute.
+  AttrBuilder &addArm64ECArgSizeAttr(uint64_t Bytes);
+
   /// This turns the unwind table kind into the form used internally in
   /// Attribute.
   AttrBuilder &addUWTableAttr(UWTableKind Kind);
Index: llvm/include/llvm/IR/Attributes.td
===================================================================
--- llvm/include/llvm/IR/Attributes.td
+++ llvm/include/llvm/IR/Attributes.td
@@ -314,6 +314,9 @@
 /// Function is a presplit coroutine.
 def PresplitCoroutine : EnumAttr<"presplitcoroutine", [FnAttr]>;
 
+/// Struct Arg size for x86 signature to generate correct thunk
+def Arm64ECArgSize : IntAttr<"arm64ec_argsize", [ParamAttr, RetAttr]>;
+
 /// Target-independent string attributes.
 def LessPreciseFPMAD : StrBoolAttr<"less-precise-fpmad">;
 def NoInfsFPMath : StrBoolAttr<"no-infs-fp-math">;
Index: llvm/include/llvm/IR/Function.h
===================================================================
--- llvm/include/llvm/IR/Function.h
+++ llvm/include/llvm/IR/Function.h
@@ -484,6 +484,12 @@
     return AttributeSets.getParamDereferenceableOrNullBytes(ArgNo);
   }
 
+  /// Extract the number of Arm64ECArgSize bytes for a parameter.
+  /// @param ArgNo Index of an argument, with 0 being the first function arg.
+  uint64_t getParamArm64ECArgSizeBytes(unsigned ArgNo) const {
+    return AttributeSets.getParamArm64ECArgSizeBytes(ArgNo);
+  }
+
   /// Determine if the function is presplit coroutine.
   bool isPresplitCoroutine() const {
     return hasFnAttribute(Attribute::PresplitCoroutine);
Index: llvm/include/llvm/IR/InstrTypes.h
===================================================================
--- llvm/include/llvm/IR/InstrTypes.h
+++ llvm/include/llvm/IR/InstrTypes.h
@@ -1814,6 +1814,18 @@
     return Attrs.getParamDereferenceableOrNullBytes(i);
   }
 
+  /// Extract the number of Arm64ECArgSize bytes for a call or
+  /// parameter.
+  uint64_t getRetArm64ECArgSizeBytes() const {
+    return Attrs.getRetArm64ECArgSizeBytes();
+  }
+
+  /// Extract the number of Arm64ECArgSize bytes for a call or
+  /// parameter.
+  uint64_t getParamArm64ECArgSizeBytes(unsigned i) const {
+    return Attrs.getParamArm64ECArgSizeBytes(i);
+  }
+
   /// Return true if the return value is known to be not null.
   /// This may be because it has the nonnull attribute, or because at least
   /// one byte is dereferenceable and the pointer is in addrspace(0).
Index: llvm/lib/AsmParser/LLParser.cpp
===================================================================
--- llvm/lib/AsmParser/LLParser.cpp
+++ llvm/lib/AsmParser/LLParser.cpp
@@ -1416,6 +1416,21 @@
     B.addAllocKindAttr(Kind);
     return false;
   }
+  case Attribute::Arm64ECArgSize: {
+    if (!EatIfPresent(lltok::kw_arm64ec_argsize))
+      return false;
+    LocTy ParenLoc = Lex.getLoc();
+    if (!EatIfPresent(lltok::lparen))
+      return error(ParenLoc, "expected '('");
+    uint64_t Bytes = 0;
+    if (parseUInt64(Bytes))
+      return true;
+    ParenLoc = Lex.getLoc();
+    if (!EatIfPresent(lltok::rparen))
+      return error(ParenLoc, "expected ')'");
+    B.addArm64ECArgSizeAttr(Bytes);
+    return false;
+  }
   default:
     B.addAttribute(Attr);
     Lex.Lex();
Index: llvm/lib/Bitcode/Reader/BitcodeReader.cpp
===================================================================
--- llvm/lib/Bitcode/Reader/BitcodeReader.cpp
+++ llvm/lib/Bitcode/Reader/BitcodeReader.cpp
@@ -2015,6 +2015,8 @@
     return Attribute::Hot;
   case bitc::ATTR_KIND_PRESPLIT_COROUTINE:
     return Attribute::PresplitCoroutine;
+  case bitc::ATTR_KIND_ARM64EC_ARG_SIZE:
+    return Attribute::Arm64ECArgSize;
   }
 }
 
Index: llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
===================================================================
--- llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
+++ llvm/lib/Bitcode/Writer/BitcodeWriter.cpp
@@ -782,6 +782,8 @@
     return bitc::ATTR_KIND_MUSTPROGRESS;
   case Attribute::PresplitCoroutine:
     return bitc::ATTR_KIND_PRESPLIT_COROUTINE;
+  case Attribute::Arm64ECArgSize:
+    return bitc::ATTR_KIND_ARM64EC_ARG_SIZE;
   case Attribute::EndAttrKinds:
     llvm_unreachable("Can not encode end-attribute kinds marker.");
   case Attribute::None:
Index: llvm/lib/IR/AttributeImpl.h
===================================================================
--- llvm/lib/IR/AttributeImpl.h
+++ llvm/lib/IR/AttributeImpl.h
@@ -257,6 +257,7 @@
   Optional<unsigned> getVScaleRangeMax() const;
   UWTableKind getUWTableKind() const;
   AllocFnKind getAllocKind() const;
+  uint64_t getArm64ECArgSizeBytes() const;
   std::string getAsString(bool InAttrGrp) const;
   Type *getAttributeType(Attribute::AttrKind Kind) const;
 
Index: llvm/lib/IR/Attributes.cpp
===================================================================
--- llvm/lib/IR/Attributes.cpp
+++ llvm/lib/IR/Attributes.cpp
@@ -222,6 +222,11 @@
   return get(Context, VScaleRange, packVScaleRangeArgs(MinValue, MaxValue));
 }
 
+Attribute Attribute::getWithArm64ECArgSizeBytes(LLVMContext &Context,
+                                                uint64_t Bytes) {
+  return get(Context, Arm64ECArgSize, Bytes);
+}
+
 Attribute::AttrKind Attribute::getAttrKindFromName(StringRef AttrName) {
   return StringSwitch<Attribute::AttrKind>(AttrName)
 #define GET_ATTR_NAMES
@@ -369,6 +374,13 @@
   return unpackVScaleRangeArgs(pImpl->getValueAsInt()).second;
 }
 
+uint64_t Attribute::getArm64ECArgSizeBytes() const {
+  assert(hasAttribute(Attribute::Arm64ECArgSize) &&
+         "Trying to get Arm64ECArgSize bytes from "
+         "non-Arm64ECArgSize attribute!");
+  return pImpl->getValueAsInt();
+}
+
 UWTableKind Attribute::getUWTableKind() const {
   assert(hasAttribute(Attribute::UWTable) &&
          "Trying to get unwind table kind from non-uwtable attribute");
@@ -472,6 +484,9 @@
         .str();
   }
 
+  if (hasAttribute(Attribute::Arm64ECArgSize))
+    return AttrWithBytesToString("arm64ec_argsize");
+
   // Convert target-dependent attributes to strings of the form:
   //
   //   "kind"
@@ -764,6 +779,10 @@
   return SetNode ? SetNode->getAllocKind() : AllocFnKind::Unknown;
 }
 
+uint64_t AttributeSet::getArm64ECArgSizeBytes() const {
+  return SetNode ? SetNode->getArm64ECArgSizeBytes() : 0;
+}
+
 std::string AttributeSet::getAsString(bool InAttrGrp) const {
   return SetNode ? SetNode->getAsString(InAttrGrp) : "";
 }
@@ -942,6 +961,12 @@
   return AllocFnKind::Unknown;
 }
 
+uint64_t AttributeSetNode::getArm64ECArgSizeBytes() const {
+  if (auto A = findEnumAttribute(Attribute::Arm64ECArgSize))
+    return A->getArm64ECArgSizeBytes();
+  return 0;
+}
+
 std::string AttributeSetNode::getAsString(bool InAttrGrp) const {
   std::string Str;
   for (iterator I = begin(), E = end(); I != E; ++I) {
@@ -1494,6 +1519,14 @@
   return getFnAttrs().getAllocKind();
 }
 
+uint64_t AttributeList::getRetArm64ECArgSizeBytes() const {
+  return getRetAttrs().getArm64ECArgSizeBytes();
+}
+
+uint64_t AttributeList::getParamArm64ECArgSizeBytes(unsigned Index) const {
+  return getParamAttrs(Index).getArm64ECArgSizeBytes();
+}
+
 std::string AttributeList::getAsString(unsigned Index, bool InAttrGrp) const {
   return getAttributes(Index).getAsString(InAttrGrp);
 }
@@ -1715,6 +1748,10 @@
   return addRawIntAttr(Attribute::VScaleRange, RawArgs);
 }
 
+AttrBuilder &AttrBuilder::addArm64ECArgSizeAttr(uint64_t Bytes) {
+  return addRawIntAttr(Attribute::Arm64ECArgSize, Bytes);
+}
+
 AttrBuilder &AttrBuilder::addUWTableAttr(UWTableKind Kind) {
   if (Kind == UWTableKind::None)
     return *this;
Index: llvm/lib/Transforms/Utils/CodeExtractor.cpp
===================================================================
--- llvm/lib/Transforms/Utils/CodeExtractor.cpp
+++ llvm/lib/Transforms/Utils/CodeExtractor.cpp
@@ -990,6 +990,7 @@
       case Attribute::ZExt:
       case Attribute::ImmArg:
       case Attribute::ByRef:
+      case Attribute::Arm64ECArgSize:
       //  These are not really attributes.
       case Attribute::None:
       case Attribute::EndAttrKinds: